clk: meson: clk-pll: fix initializing the old rate (fallback) for a PLL

The "rate" parameter in meson_clk_pll_set_rate() contains the new rate.
Retrieve the old rate with clk_hw_get_rate() so we don't inifinitely try
to switch from the new rate to the same rate again.

Fixes: 7a29a869 ("clk: meson: Add support for Meson clock controller")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20201226121556.975418-2-martin.blumenstingl@googlemail.com