cxgb3: More flexible support for PHY interrupts.

Do not require PHY interrupts to be connected to GPIs in ascending order.
Base interrupt availability both on PHYs supporting them and on GPIs being
hooked up.  Allows boards to specify interrupt GPIs though the PHYs don't
use them.

Remove spurious PHY interrupts due to clearing T3DBG interrupts before
setting their polarity.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>