s390/irq: use assignment instead of cast

Change struct ext_code to contain a union which allows to simply
assign the int_code instead of using a cast.

In order to keep the patch small the anonymous union is embedded
within the existing struct instead of changing the struct ext_code to
a union.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>