bnxt_en: Add coalescing support for tx rings.

When tx and rx rings don't share the same completion ring, tx coalescing
parameters can be set differently from the rx coalescing parameters.
Otherwise, use rx coalescing parameters on shared completion rings.

Adjust rx coalescing default values to lower interrupt rate.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>