mmc: cavium: Use module_pci_driver to simplify the code

Use the module_pci_driver() macro to make the code simpler
by eliminating module_init and module_exit calls.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>