net: stmmac: Rework stmmac_rx()

This looks over-engineered. Let's use some helpers to get the buffer
length and hereby simplify the stmmac_rx() function. No performance drop
was seen with the new implementation.

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