gianfar: Factor out gfar_init_bds() from gfar_alloc_skb_resources()

After hibernation we want to just reinitialize BDs, no need to allocate
anything. So, factor out BDs initialization code from
gfar_alloc_skb_resourses().

Also, teach gfar_init_bds() to reuse already allocated RX SKBs, i.e.
just call gfar_init_rxbdp() if a SKB was already allocated and mapped.

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