aspeed: ast2500: Fix H-PLL and M-PLL clock rate calculation

Fix H-PLL and M-PLL rate calculation in ast2500 clock driver.
Without this fix, valid setting can lead to division by zero
when requesting the rate of H-PLL or M-PLL clocks.

Signed-off-by: Maxim Sloyko <maxims@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>