alx: fix ethtool support code

A number of places treated features wrongly, listing not-supported
features instead of supported ones. Also, the get_drvinfo ethtool
callback isn't needed, and alx_get_pauseparam can be simplified.

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>