crypto: caam - fix offset field in hw sg entries

The offset field is 13 bits wide; make sure we don't overwrite more than
that in the caam hardware scatter gather structure.

Signed-off-by: Cristian Stoica <cristian.stoica@freescale.com>
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>