clk: mtk: use IS_ERR_VALUE() to check rate return values

clk: mtk: use IS_ERR_VALUE() to check rate return values

Replace casting with long to IS_ERR_VALUE() macro to check for error
return values from rate calculation functions. This is the recommended
way to check the return value from clock rate functions.

Signed-off-by: David Lechner <dlechner@baylibre.com>