sfxge: support Rx checksum offloads disabling

We can't disable it in HW, but we can ignore result.
Discard Rx descriptor checksum flags if Rx checksum offload is off.

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D2544