crypto: omap-sham - fix very small data size handling

With very small data sizes, the whole data can end up in the xmit
buffer. This code path does not set the sg_len properly which causes the
core dma framework to crash. Fix by adding the proper size in place.
Also, the data length must be a multiple of block-size, so extend the
DMA data size while here.

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