crypto: img-hash - remove need for error return variable ret

The assignment to error return variable ret and then the jump to
an error exit path can be simplified by just returning the error
return at the failure point. This allows variable ret and the
error return path to be removed. This cleans up a static analysis
warninng that variable ret is being assigned (value never being
used) and being re-assigned later.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>