CLK: vt8500: Fix SDMMC clk special cases

This patch adds some additional handling for the SDMMC special case
in round_rate and set_rate which results in invalid divisor messages
at boot time.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Mike Turquette <mturquette@linaro.org>