MIPS: ath79: Use local ref clock rate in ar934x_get_pll_freq

The reference clock rate is passed in the first
argument of the function. Use that instead of the
rate of the global ath79_ref_clk variable.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5779/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>