crypto: omap-sham - Map SG pages if they are HIGHMEM before accessing

HIGHMEM pages may not be mapped so we must kmap them before accessing.
This resolves a random OOPs error that was showing up during OpenSSL SHA tests.

Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>