target-arm: Reinsert missing return statement in ARM mode SRS decode

Since patch
   81465888c5306cd94abb9847e560796fd13d3c2f
   target-arm: factor out handling of SRS instruction
the ARM mode SRS instruction has not worked in QEMU.

The problem is a missing return directive that was removed in the
refactoring, so after decoding the instruction, qemu would fall through
to generate an UNDEF exception for an illegal instruction.

Signed-off-by: Peter Chubb <peter.chubb@nicta.com.au>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
