usb: gadget: use module_pci_driver

This patch converts the drivers in drivers/usb/gadget/* to use
module_pci_driver() macro which makes the code smaller and a bit simpler.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Thomas Dahlmann <dahlmann.thomas@arcor.de>
Cc: Xiaochen Shen <xiaochen.shen@intel.com>
Cc: Pavankumar Kondeti <pkondeti@codeaurora.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>