mmc: sdhci: Allow override of get_cd() called from sdhci_request()

Drivers may need to provide their own get_cd() mmc host op, but
currently the internals of the current op (sdhci_get_cd()) are
provided by sdhci_do_get_cd() which is also called from
sdhci_request().

To allow override of the get_cd functionality, change sdhci_request()
to call ->get_cd() instead of sdhci_do_get_cd().

Note, in the future the call to ->get_cd() will likely be removed
from sdhci_request() since most drivers don't need actually it.
However this change is being done now to facilitate a subsequent
bug fix.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@vger.kernel.org # v4.4+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>