accel/amdxdna: Fix NULL pointer dereference of mgmt_chann

accel/amdxdna: Fix NULL pointer dereference of mgmt_chann

mgmt_chann may be set to NULL if the firmware returns an unexpected
error in aie2_send_mgmt_msg_wait(). This can later lead to a NULL
pointer dereference in aie2_hw_stop().

Fix this by introducing a dedicated helper to destroy mgmt_chann
and by adding proper NULL checks before accessing it.

Fixes: b87f920b9344 ("accel/amdxdna: Support hardware mailbox")
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/20260226213857.3068474-1-lizhi.hou@amd.com