mmc: core: Fix possible NULL pointer dereference

Fix possible NULL pointer dereference within cd-gpio IRQ
handler mmc_gpio_cd_irqt(), which can potentially happen
if the card detect IRQ gets triggered before host->ops
are initialized.

CRs-fixed: 628836
Change-Id: I2f47717d338c5c7f8e7a994c28bb5c30b30286c6
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>