crypto: mxc-scc - signedness bugs in mxc_scc_ablkcipher_req_init()

->src_nents and ->dst_nents are unsigned so they can't be less than
zero.  I fixed this by introducing a temporary "nents" variable.

Fixes: d293b640

 ('crypto: mxc-scc - add basic driver for the MXC SCC')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>