uwb: fix uwb_dev_unlock() missed at an error path in uwb_rc_cmd_async()

There is the only path in uwb_rc_cmd_async() where rc->uwb_dev is left unlocked.

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

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