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
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35382