net: stmmac: Prepare to add Split Header support

In order to add Split Header support, stmmac_rx() needs to take into
account that packet may be split accross multiple descriptors.

Refactor the logic of this function in order to support this scenario.

Changes from v2:
	- Fixup if condition detection (Jakub)
	- Don't stop NAPI with unfinished packet (Jakub)
	- Use napi_alloc_skb() (Jakub)

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>