crypto: omap-sham - convert driver logic to use sgs for data xmit

Currently, the internal buffer has been used for data transmission. Change
this so that scatterlists are used instead, and change the driver to
actually use the previously introduced helper functions for scatterlist
preparation.

This patch also removes the old buffer handling code which is no longer
needed.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>