gianfar: Factor out RX BDs initialization from gfar_new_rxbdp()

We want to just reinitialize RX BDs after hibernation, no need to
map the skb->data again. So let's factor gfar_init_rxbdp() out of
gfar_new_rxbdp().

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>