crypto: Validate return values from CRYPTODEV_PROCESS()

Errors are always handled by the completion callback, so we should check
that they're not also passed back to the caller.

No functional change intended.

Reviewed by: kp, mav, jhb
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 99df914899f56efe63afd9e0fef79148fa6ca162)