s390/raw3270: remove BUG_ON in raw3270_request_reset()

WARN_ON_ONCE if list is not empty, and return an error code
instead.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>