usb: gadget: use $(srctree) instead of $(PWD) for includes

Using $(PWD) breaks builds when make was invoked from outside
of the kernel tree.
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/Makefile

@@ -3,7 +3,7 @@
 #
 subdir-ccflags-$(CONFIG_USB_GADGET_DEBUG)	:= -DDEBUG
 subdir-ccflags-$(CONFIG_USB_GADGET_VERBOSE)	+= -DVERBOSE_DEBUG
-ccflags-y				+= -I$(PWD)/drivers/usb/gadget/udc
+ccflags-y				+= -I$(srctree)/drivers/usb/gadget/udc
 
 obj-$(CONFIG_USB_LIBCOMPOSITE)	+= libcomposite.o
 libcomposite-y			:= usbstring.o config.o epautoconf.o

drivers/usb/gadget/function/Makefile

@@ -2,8 +2,8 @@
 # USB peripheral controller drivers
 #
 
-ccflags-y			:= -I$(PWD)/drivers/usb/gadget/
-ccflags-y			+= -I$(PWD)/drivers/usb/gadget/udc/
+ccflags-y			:= -I$(srctree)/drivers/usb/gadget/
+ccflags-y			+= -I$(srctree)/drivers/usb/gadget/udc/
 
 # USB Functions
 usb_f_acm-y			:= f_acm.o

drivers/usb/gadget/legacy/Makefile

@@ -2,9 +2,9 @@
 # USB gadget drivers
 #
 
-ccflags-y			:= -I$(PWD)/drivers/usb/gadget/
-ccflags-y			+= -I$(PWD)/drivers/usb/gadget/udc/
-ccflags-y			+= -I$(PWD)/drivers/usb/gadget/function/
+ccflags-y			:= -I$(srctree)/drivers/usb/gadget/
+ccflags-y			+= -I$(srctree)/drivers/usb/gadget/udc/
+ccflags-y			+= -I$(srctree)/drivers/usb/gadget/function/
 
 g_zero-y			:= zero.o
 g_audio-y			:= audio.o