drivers: entropy: nrf5: Fix hardware state management

The entropy_nrf5_get_entropy_isr(...,  ENTROPY_BUSYWAIT)
unconditionally started and stopped hardware random number generator
disrupting management of random data pools implemented by the driver.

Moreover, simultaneous call to mentioned function from different
priorities led to hangup of lower priority call as hardware was
stopped by higher priority call while the other one waited for data.

This commit solves mentioned problems by moving the responsibility
for stopping hardware to the interrupt handler. As result hardware
is stopped only when there is no blocking calls to the
entropy_nrf5_get_entropy_isr() as well as all random data pools
are full.

This commit also fixes interrupt management in the mentioned
function, as previous implementation unconditionally enabled
interrupt causing problems similar to the described above.

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