mailbox: Fix NULL message support in mbox_send_message()

mailbox: Fix NULL message support in mbox_send_message()

The active_req field serves double duty as both the "is a TX in
flight" flag (NULL means idle) and the storage for the in-flight
message pointer. When a client sends NULL via mbox_send_message(),
active_req is set to NULL, which the framework misinterprets as
"no active request". This breaks the TX state machine by:

 - tx_tick() short-circuits on (!mssg), skipping the tx_done
   callback and the tx_complete completion
 - txdone_hrtimer() skips the channel entirely since active_req
   is NULL, so poll-based TX-done detection never fires.

Fix this by introducing a MBOX_NO_MSG sentinel value that means
"no active request," freeing NULL to be valid message data. The
sentinel is defined in the subsystem-internal mailbox.h so that
controller drivers within drivers/mailbox/ can reference it, but
it is not exposed to clients outside the subsystem.

Fifteen in-tree callers send NULL (doorbell-style IPCs on Qualcomm,
Tegra, TI, Xilinx, i.MX, SCMI, and PCC platforms). All were
audited for regression:

 - Most already work around the bug via knows_txdone=true with a
   manual mbox_client_txdone() call, making the framework's
   tracking irrelevant. These are unaffected.

 - Poll-based callers (Xilinx zynqmp/r5) are strictly better off:
   the poll timer now correctly detects NULL-active channels
   instead of silently skipping them.

 - irq-qcom-mpm.c was a pre-existing bug -- the only Qualcomm
   caller that omitted the knows_txdone + mbox_client_txdone()
   pattern. Fixed in a companion commit ("irqchip/qcom-mpm: Fix
   missing mailbox TX done acknowledgment").

 - No caller sets both a tx_done callback and sends NULL, nor
   combines tx_block=true with NULL sends, so the newly reachable
   callback/completion paths are never exercised.

Also update tegra-hsp's flush callback, which directly inspects
active_req to wait for the channel to drain: the old "!= NULL"
check becomes "!= MBOX_NO_MSG", otherwise flush spins until
timeout since the sentinel is non-NULL.

The only tradeoff is that 'MBOX_NO_MSG' can not be used as a message
by clients.

Reported-by: Joonwon Kang <joonwonkang@google.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>