target-arm: Fix CP15 based WFI

target-arm: Fix CP15 based WFI

The coprocessor register rework broke cp15 based WFI instructions.
We incorrectly fall through the normal register write case, which
incorrectly adds a forced block termination.  We've already done
a special version of this (DISAS_WFI), so return immediately.

Signed-off-by: Paul Brook <paul@codesourcery.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>