usb: move usb/usb-common.c to usb/common/usb-common.c

Since we will have more usb-common things, and it will let
usb-common.c be larger and larger, we create a folder named usb/common
for all usb common things.

Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>