usb: Rename usb-common.c

In the next commit, we will want the usb-common module to be composed of
two object files. Since Kbuild cannot "append" another object to an
existing one, we need to rename usb-common.c to something
else (common.c) and create usb-common.o by linking the wanted objects
together. Currently, usb-common.o comprises only common.o.

Signed-off-by: Michal Sojka <[e-mail]>
Acked-by: Felipe Balbi <[e-mail]>
Tested-by: Felipe Balbi <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/common/Makefile

···
 # Makefile for the usb common parts.
 #
 
-obj-$(CONFIG_USB_COMMON) += usb-common.o
+obj-$(CONFIG_USB_COMMON)	  += usb-common.o
+usb-common-y			  += common.o
+
 obj-$(CONFIG_USB_OTG_FSM) += usb-otg-fsm.o