mmc: dw_mmc: Don't try to enable the CD until we're sure we're not deferring

If dw_mci_init_slot() returns that we got a probe deferral then it may
leave slot->mmc as NULL.  That will cause dw_mci_enable_cd() to crash
when it calls mmc_gpio_get_cd().

Fix this by moving the call of dw_mci_enable_cd() until we're sure
that we're good.  Note that if we have more than one slot and one
defers (but the others don't) things won't work so well.  ...but
that's not a new thing and everyone has already agreed that multislot
support ought to be removed from dw_mmc eventually anyway since it is
unused, untested, and you can see several bugs like this by inspecting
the code.

Fixes: bcafaf5470f0 ("mmc: dw_mmc: Only enable CD after setup and only if needed")
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>