crypto: omap-sham - correct dma burst size

Each cycle of SHA512 operates on 32 data words where as
SHA256 operates on 16 data words. This needs to be updated
while configuring DMA channels. Doing the same.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>