sky2: Factor out code to calculate packet sizes

Move code to calculate receive threshold and packet size out of
sky2_rx_start() so that is can be called from elsewhere easily.

Signed-off-by: Mike McCormack <mikem@ring3k.org>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>