staging: rts5208: Use common pci_get_bus_and_slot() instead of private one

We already have a inline pci_get_bus_and_slot() in include/linux/pci.h,
Use it instead of local one.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>