octeon_ep: initialize mbox mutexes

octeon_ep: initialize mbox mutexes

The two mbox-related mutexes are destroyed in octep_ctrl_mbox_uninit(),
but the corresponding mutex_init calls were missing.
A "DEBUG_LOCKS_WARN_ON(lock->magic != lock)" warning was emitted with
CONFIG_DEBUG_MUTEXES on.

Initialize the two mutexes in octep_ctrl_mbox_init().

Fixes: 577f0d1b1c5f ("octeon_ep: add separate mailbox command and response queues")
Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20230729151516.24153-1-mschmidt@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>