cxgb3 - Update rx coalescing length

Reduce Rx coalescing length to 12288
Large bursts from the adapter to the host create back pressure
on the chip. Reducing the burst size avoids the issue.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>