clk: rockchip: Prevent calculating mmc phase if clock rate is zero

The MMC sample and drv clock for rockchip platforms are derived from
the bus clock output to the MMC/SDIO card. So it should never happens
that the clk rate is zero given it should inherits the clock rate from
its parent. If something goes wrong and makes the clock rate to be zero,
the calculation would be wrong but may still make the mmc tuning process
work luckily. However it makes people harder to debug when the following
data transfer is unstable.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>