cxgbe(4): Do not panic when tx is called with invalid checksum requests.

There is no need to panic in if_transmit if the checksums requested are
inconsistent with the frame being transmitted. This typically indicates
that the kernel and driver were built with different INET/INET6 options,
or there is some other kernel bug. The driver should just throw away
the requests that it doesn't understand and move on.

MFC after: 1 week
Sponsored by: Chelsio Communications