crypto: omap-sham - fix buffer handling for split test cases

Current buffer handling logic fails in a case where the buffer contains
existing data from previous update which is divisible by block size.
This results in a block size of data to be left missing from the sg
list going out to the hw accelerator, ending up in stalling the
crypto accelerator driver (the last request never completes fully due
to missing data.)

Fix this by passing the total size of the data instead of the data size
of current request, and also parsing the buffer contents within the
prepare request handling.

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