staging: mei: unlock dev->device_lock mutex on error path in mei_open()

mei_open() acquires dev->device_lock mutex and try to allocate mei_cl,
but if the allocation fails it goes to return statement. As a result
dev->device_lock left locked.

The patch fixes goto to unlock dev->device_lock mutex on this path.

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>