crypto: keembay - Add missing check after sg_nents_for_len()

sg_nents_for_len() returns an int which is negative in case of error.

Fixes: 472b0444 ("crypto: keembay - Add Keem Bay OCS HCU driver")
Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>