crypto: omap - switch from scatter_walk to plain offset

The omap driver was using struct scatter_walk, but only to maintain an
offset, rather than iterating through the virtual addresses of the data
contained in the scatterlist which is what scatter_walk is intended for.
Make it just use a plain offset instead.  This is simpler and avoids
using struct scatter_walk in a way that is not well supported.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>