net: stmmac: convert to hw_features

This also removes TSO as it's made fully in software --- better to leave this
to networking core.

If the MAC features can be detected at probe time and not at open, then
stmmac_fix_features could be simplified by limiting hw_features. That's
also better for users as they don't see offloads being togglable but
never turned on.

Redundant fallbacks for TX csum are removed as it's already handled
by network core.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>