crypto: inside-secure/eip93 - Correctly handle return of for sg_nents_for_len

Fix smatch warning for sg_nents_for_len return value in Inside Secure
EIP93 driver.

The return value of sg_nents_for_len was assigned to an u32 and the
error was ignored and converted to a positive integer.

Rework the code to correctly handle the error from sg_nents_for_len to
mute smatch warning.

Fixes: 9739f5f9 ("crypto: eip93 - Add Inside Secure SafeXcel EIP-93 crypto engine support")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>