accel/amdxdna: Clear mailbox interrupt register during channel creation

accel/amdxdna: Clear mailbox interrupt register during channel creation

The mailbox interrupt register is not always cleared when a mailbox channel
is created. This can leave stale interrupt states from previous operations.

Fix this by explicitly clearing the interrupt register in the mailbox
channel creation function.

Fixes: b87f920b9344 ("accel/amdxdna: Support hardware mailbox")
Reviewed-by: Maciej Falkowski <maciej.falkowski@linux.intel.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/20251107181115.1293158-1-lizhi.hou@amd.com