clk: meson: clk-pll: make "ret" a signed integer

The error codes returned by meson_clk_get_pll_settings() are all
negative. Make "ret" a signed integer in meson_clk_pll_set_rate() to
make it match with the clk_ops.set_rate API as well as the data type
returned by meson_clk_get_pll_settings().

Fixes: 8eed1db1 ("clk: meson: pll: update driver for the g12a")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20201226121556.975418-3-martin.blumenstingl@googlemail.com