crypto: marvell/cesa - const-ify argument to mv_cesa_get_op_cfg()

mv_cesa_get_op_cfg() does not write to its argument, it only reads.
So, let's make it const.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>