target/arm: Fix offset scaling for LD_zprr and ST_zprr

target/arm: Fix offset scaling for LD_zprr and ST_zprr

The scaling should be solely on the memory operation size; the number
of registers being loaded does not come in to the initial computation.

Cc: qemu-stable@nongnu.org (3.0.1)
Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 50ef1cbf31caad21019ae6fa8036ed6f29244ba5)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>