RAID/s390: remove invalid 'r' inline asm operand modifier

gcc silently ignores unsupported inline asm operand modifiers, effectively
turning '%r0' into '%0', but upcoming clang 9 complains about them:
lib/raid6/s390vx8.c:63:16: error: invalid operand in inline asm: 'VLM $2,$3,0,${1:r}'
        asm volatile ("VLM %2,%3,0,%r1"
                      ^

Clean up what look like a typo 'r' inline asm operand modifier usage.

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