IPoIB: Remove TX moderation settings from ethtool support

As of commit f56bcd80 ("IPoIB: Use separate CQ for UD send
completions"), there are no TX interrupts.  Change the ethtool code
not to report TX moderation settings, so users will not be misled to
think they can control TX interrupt moderation.  Pointed out by Alex
Vainman <alexv@voltaire.com>

Signed-off-by: Or Gerlitz <ogerlitz@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>