remoteproc: imx_dsp_rproc: Skip RP_MBOX_SUSPEND_SYSTEM when mailbox TX channel is...

remoteproc: imx_dsp_rproc: Skip RP_MBOX_SUSPEND_SYSTEM when mailbox TX channel is uninitialized

Firmwares that do not use mailbox communication (e.g., the hello_world
sample) leave priv->tx_ch as NULL. The current suspend logic
unconditionally sends RP_MBOX_SUSPEND_SYSTEM, which is invalid without
an initialized TX channel.

Detect the no_mailboxes case early and skip sending the suspend
message. Instead, proceed directly to the runtime PM suspend path,
which is the correct behavior for firmwares that cannot respond to
mailbox requests.

Signed-off-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Link: https://lore.kernel.org/r/20251204122825.756106-1-iuliana.prodan@oss.nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>