mfd: Use module_pci_driver

This patch converts the drivers in drivers/mfd/* 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: Andres Salomon <dilinger@queued.net>
Cc: Ira W. Snyder <iws@ovro.caltech.edu>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Denis Turischev <denis@compulab.co.il>
Cc: Harald Welte <HaraldWelte@viatech.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>