hwrng: imx-rngc - simpler check for available random bytes

The "level" field in the status register contains the number of random
bytes that are available in the FIFO.  Use GENMASK to extract this field.
We only want to check if level is 0 or if we can read another byte.
There's no need for the shift or the level variable.

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