imx8mq: fix FRAC_PLL_REFCLK_SEL_MASK

Coverity reported dead code, however it is FRAC_PLL_REFCLK_SEL_MASK
was wrongly set.

Reported-by: Coverity 10045172
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>