net: ethernet: stmmac: Clean up dead code

Many macros in dwmac-ipq806x are unused and should be removed.
Moreover gmac->id is an unsigned variable and therefore checking
whether it is less than zero is redundant.

Signed-off-by: Christos Gkekas <chris.gekas@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>