s390/con3270: simplify update flags

Make TTY3270_UPDATE_ALL the sum of all TTY3270_* flags, so we
don't need any special handling for it.

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>