crypto: caam - Fix incorrect size when DMA unmapping buffer

The CAAM driver uses two data buffers to store data for a hashing operation,
with one buffer defined as active. This change forces switching of the
active buffer when executing a hashing operation to avoid a later DMA unmap
using the length of the opposite buffer.

Signed-off-by: Victoria Milhoan <vicki.milhoan@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>