s390/lgr: use simple assignment instead of memcpy

It is quite pointless to use memcpy to copy two bytes, besides that
this construct will also partially remove type and size sanity checks.

Therefore simply use an assignment.

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