stmmac: dwmac-ipq806x: move ipq806x_gmac_fix_mac_speed function

Move ipq806x_gmac_fix_mac_speed in preparation for turning
the setup glue callback in a proper probe function.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>