s390/traps: simplify data exception handler

Simplify conditions and remove unnecessary variable in data exception
handler.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>