mfd: rtsx: Fix oops when rtsx_pci_sdmmc is not probed

If rtsx_pci_sdmmc is not probed, function pointer pcr->slots[].card_event
will point to NULL, and thus rtsx_pci_card_detect will reference a NULL
pointer.
Check card_event pointer before referencing it can avoid kernel panic.

Signed-off-by: Wei WANG <wei_wang@realsil.com.cn>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>