s390/raw3270: split up raw3270_activate_view()

move the core processing to __raw3270_activate_view() to
reduce the required if/else blocks and indentiion levels.

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>