s390/tty3270: add AID defines

Use AID_* instead of hex numbers to make the code a bit
easier to read. also convert the if/else blocks to a
switch statement in tty3270_read_tasklet().

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