accel/amdxdna: Guard management mailbox channel cleanup against NULL pointer

accel/amdxdna: Guard management mailbox channel cleanup against NULL pointer

The management mailbox channel cleanup helpers can be called from
error handling paths when mgmt_chann has already been destroyed.

Add NULL checks to xdna_mailbox_free_channel() and
xdna_mailbox_stop_channel() so the cleanup path safely returns instead
of dereferencing a NULL mailbox channel pointer.

Fixes: b87f920b9344 ("accel/amdxdna: Support hardware mailbox")
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Max Zhen <max.zhen@amd.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/20260416201106.1046072-1-lizhi.hou@amd.com