clk: stm32: Fix a signedness issue in clk_stm32_composite_determine_rate()

The divider_ro_round_rate() function could potentially return -EINVAL on
error but the error handling doesn't work because "rate" is unsigned.
It should be a type long.

Fixes: 06ed0fc0

 ("clk: stm32: composite: Switch to determine_rate")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/d9a78453-9b40-48c1-830e-00751ba3ecb8@kili.mountain
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>