drivers: ieee802154_nrf5: fix delayed operation accuracy

Align clock accuracy used in CSL calculations to the value in platform.
This is recomended setting for devices working in wide range
temperature. Nevertheless it can be profiled in end product to decrease
CSL window duration and finally the power consumption.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>