mmc: sdhci-pltfm: remove the unneeded check of disabled device

Since commit cd1e6504

 ("of/device: Don't register disabled
devices"), the disabled device will not be registered at all. So
we don't need to do the check again in the platform device driver.

And the check in the current code is useless even if we really
run into a disabled device. In this case, it just doesn't parse
the dtb for the infos such as quirks or clock, but it will continue
to try to init the disabled device after that check. So just remove it.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>