crypto: ansi_cprng - Fix test in get_prng_bytes

size_t nbytes cannot be less than 0 and the test was redundant.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>