modules: hal_nordic: nrfs: fix buffer used in nrfs send

In function nrfs_backend_send_work tx_data buffer was
used in function ipc_service_send instead of
data_to_send. This is fixed and also tx_data and rx_data
are moved to coresponding functions preventing such
issues in future.

Signed-off-by: Lukasz Stepnicki <lukasz.stepnicki@nordicsemi.no>