s390/nmi: Simplify ptregs setup

The low level machine check handler code fills the ptregs structure
partially with the register contents present at machine check handler
entry and partially with contents from the machine check save area.

In case of a machine check the contents of all general purpose
registers are saved by the CPU to the machine check save area.
Therefore simplify the code and fill the ptregs structure by only
using the machine check save area as source.

Signed-off-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>