tg3: Update ethtool set_settings error checks

The ethtool interface has acquired some new enumerations since the
tg3 driver's tg3_set_settings() error checking code was written.  The
error checking code is no longer complete.  This patch rewrites the
error checking so that it is future-proofed.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>