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

Since patch
   81465888
   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: NPeter Chubb <peter.chubb@nicta.com.au>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target-arm/translate.c

@@ -6762,6 +6762,7 @@ static void disas_arm_insn(CPUARMState * env, DisasContext *s)
             }
             ARCH(6);
             gen_srs(s, (insn & 0x1f), (insn >> 23) & 3, insn & (1 << 21));
+            return;
         } else if ((insn & 0x0e50ffe0) == 0x08100a00) {
             /* rfe */
             int32_t offset;