cxgb: replace r273280 with a more comprehensive fix.

Poll for link state when the link is down, even for interrupt capable
PHYs.

Allow PHYs to report a dubious "partial" link. If this state is seen 3
consecutive times (each check is ~1s apart) then reset the PHY. This is
a workaround for a situation where repeatedly toggling the link from the
peer gets the AEL2005 PHY into a state where it never establishes a PCS
block lock even when everything is in order.

MFC after: 1 week