crypto: omap-sham - fix SW fallback HMAC handling for omap2/omap3

If software fallback is used on older hardware accelerator setup (OMAP2/
OMAP3), the first block of data must be purged from the buffer. The
first block contains the pre-generated ipad value required by the HW,
but the software fallback algorithm generates its own, causing wrong
results.

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