net: stmmac: add set_mac to the stmmac_ops

Two different set_mac functions exists but stmmac_dwmac4_set_mac() is
only used for enabling and never for disabling.
So on dwmac4, the MAC RX/TX is never disabled.

This patch add a generic function pointer set_mac() to stmmac_ops and
replace all call to stmmac_set_mac/stmmac_dwmac4_set_mac by a call to
this pointer.

Since dwmac4_ops is const, set_mac cannot be modified after, and so dwmac4_ops
is duplioacted like dwmac4_dma_ops.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>