imx: mx6ul/sx: Fix issue in LCDIF clock dividers calculation

The checking with max frequency supported is not correct, because the temp
is calculated by max pre and post dividers. We can decrease any divider to
meet the max frequency limitation. Actually, the calculation below the codes
is doing this way to find best pre and post dividers.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>