s390/nmi: fix order of register validation

When validating register contents first validate control registers
since these control the availability of features later being
validated.
For example the control register 0 should be validated first, before
the additional floating point (AFP) registers are validated, since
control register 0 contains the AFP-register control bit.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>