mmc: via-sdmmc: Use module_pci_driver

module_pci_driver makes the code simpler by eliminating
module_init and module_exit calls.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Chris Ball <cjb@laptop.org>