modules: hal_nordic: nrf_802154: lengthen serialization ring buffer

This commit increases the length of ring buffer that holds serialized
nRF 802.15.4 API calls so that it can simultaneously store all
notifications the driver is capable of issuing. Currently that's not the
case, which creates a possibility of the serialization buffers running
out while the driver is issuing notifications.

Signed-off-by: Jędrzej Ciupis <jedrzej.ciupis@nordicsemi.no>