staging: lirc: fix mismatch in mutex lock-unlock in imon_probe

If kzalloc failed for allocation of context, goto alloc_status_switch
leads to unlock of unacquired driver_lock. The patch moves
mutex_lock(&driver_lock) before allocation of context.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>