gianfar: Split allocation and initialization steps out of startup_gfar()

Two new functions implemented: gfar_alloc_skb_resources() and
gfar_init_mac(). We'll use gfar_init_mac() for restoring after
hibernation.

The patch just moves the code around, there should be no functional
changes.

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