hw/arm_sysctl.c: Add missing 'break' statements

hw/arm_sysctl.c: Add missing 'break' statements

Add some break statements that were accidentally omitted
from some cases of arm_sysctl_write(). The omission was
harmless because in both cases the following case did
an immediate break, but adding the breaks explicitly
placates static analysers and avoids weird behaviour if
the following register is ever implemented as something
other than a no-op.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>