s390/nmi: Remove duplicate get_lowcore() calls

Assign the output from get_lowcore() to a local variable,
so the code is easier to read.

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