Fix fallout from r292180 (Dec 2015)... ensure that every driver which has
a DRIVER_MODULE() referencing mmc_driver has a MODULE_DEPEND() on mmc. This
is because the kernel linker only searches for symbols in dependent modules,
so loading sdhci_pci (and other bus-flavors of sdhci) would fail when mmc
was not compiled into the kernel (even if you hand-loaded mmc first).

(Thanks to jilles@ for providing the vital clue about the kernel linker.)