s390/irq: reduce size of external interrupt handler hash array

Change the hash algorithm a bit so it produces only values in the
range of 0..31.
This allows to reduce the size of the external interrupt handler hash
array even further while making sure that each of the known interrupt
sources keeps its unique hash with the slightly modified algorithm:

0x1004 --> 12
0x1201 --> 10
0x1202 --> 11
0x1406 --> 16
0x1407 --> 17
0x2401 --> 19
0x2603 --> 22
0x4000 --> 0

This also means that the entire array now fits into exactly one cache
line; so add a proper align statement as well.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>