modules: tf-m: Fix thread preemption toggling

Fixes an issue where thread preemption was not being disabled
correctly, failing to taking into account MetaIRQ, which can be
used to preempt any cooperative thread.

The updated code sets the current thread to `K_HIGHEST_THREAD_PRIO`
before calling the secure function, and restores the thread priority
level once secure execution has terminated, allowing the thread
to be preempted once again.

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>