net: stmmac: Enable stmmac main clock when probing hardware

The stmmac driver does not enable the main clock during the probe phase.
If the clock was not enabled by the boot loader or was disabled by the
kernel, hardware features and the MDIO bus would not be probed properly.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>