arch: arm: aarch32: cortex-m: fix logic for detecting guard violation

We fix (by inverting) the logic of the IS_MPU_GUARD_VIOLATION
macro, with respect to the value of the supplied 'fault_addr'.
We shall only be inspecting the fault_addr value if it is not
set to -EINVAL.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>