drivers: entropy: nrf5: Remove entropy_nrf_get_entropy_isr() function

The entropy_nrf_get_entropy_isr(), which is specific to this driver,
is in fact equivalent of generic entropy_get_entropy_isr(..., 0).

This commit removes the entropy_nrf_get_entropy_isr() function
and replaces its usage by call to generic entropy API.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>