modules: hal_nordic: nrfs: Optimize communication

It is not needed to use always the system work queue to send a
message over IPC. In thread context IPC service can be called
directly. It speeds up the communication and allows to use nrfs
from the system work queue. Legacy approach could easily lead
to the deadlock if user would call nrfs from work queue and
pend on semaphore until response is received.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>