hwrng: imx-rngc - simplify interrupt mask/unmask

Use a simpler approach for masking / unmasking the rngc interrupt:
The interrupt is unmasked while self-test is running and when the rngc
driver is used by the hwrng core.

Mask the interrupt again when self test is finished, regardless of
self test success or failure.

Unmask the interrupt in the init function. Add a cleanup function where
the rngc interrupt is masked again.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>