KVM: s390: Get rid of ar_t

sparse with __CHECK_ENDIAN__ shows that ar_t was never properly
used across KVM on s390. We can now:
- fix all places
- do not make ar_t special
Since ar_t is just used as a register number (no endianness issues
for u8), and all other register numbers are also just plain int
variables, let's just use u8, which matches the __u8 in the userspace
ABI for the memop ioctl.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>