net: broadcom: bcm4908_enet: support TX interrupt

It appears that each DMA channel has its own interrupt and both rings
can be configured (the same way) to handle interrupts.

1. Make ring interrupts code generic (make it operate on given ring)
2. Move napi to ring (so each has its own)
3. Make IRQ handler generic (match ring against received IRQ number)
4. Add (optional) support for TX interrupt

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>