eth: fbnic: Add basic Tx handling

Handle Tx of simple packets. Support checksum offload and gather.
Use .ndo_features_check to make sure packet geometry will be
supported by the HW, i.e. we can fit the header lengths into
the descriptor fields.

The device writes to the completion rings the position of the tail
(consumer) pointer. Read all those writebacks, obviously the last
one will be the most recent, complete skbs up to that point.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://patch.msgid.link/172079942464.1778861.17919428039428796180.stgit@ahduyck-xeon-server.home.arpa


Signed-off-by: Jakub Kicinski <kuba@kernel.org>