net/stmmac: fix issue of clk_get for Loongson1B.

When getting clock, give a chance to the CPUs without DT support,
which use Common Clock Framework, such as Loongson1B.

Signed-off-by: Kelvin Cheung <keguang.zhang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>