Install a handler for firmware work request error messages.

If a driver sends an malformed or disallowed work request, the firmware
responds with a work request error. Previously the driver treated this is
as an unexpected message and panicked. Now it decodes the error message
to aid in debugging.

Reviewed by: np (older version)
MFC after: 1 month
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D6950