mmc: sdhci-esdhc: Only force clock output if slot supports 1.8v

For i.MX6Q we set ESDHC_VENDOR_SPEC_FRC_SDCLK_ON to force clock
output to support SD3.0.  If the host does not support the 1.8v
signalling required for SD3.0 then don't twiddle these bits and
mimic the functionality of the standard esdhc_set_clk().