hwrng: pic32 - enable TRNG only while it's used

The probe function enables the TRNG hardware before registering the
driver. If registration fails, probe returns an error, but the TRNG
remains enabled.

Define init and cleanup functions, enable and disable the hardware there.

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