net: stmmac: Rework stmmac_rx()

[ Upstream commit 88ebe2cf ]

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>
Stable-dep-of: fa02de9e ("net: stmmac: fix rx budget limit check")
Signed-off-by: Sasha Levin <sashal@kernel.org>