cnic: Add function pointers to arm IRQ for different devices.

This will make it easier to exit IRQ loop and re-arm IRQ on devices that
don't support FCoE.

Reviewed-by: Eddie Wai <eddie.wai@broadcom.com>
Reviewed-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>