usb: gadget: remove $(PWD) in ccflags-y

usb: gadget: remove $(PWD) in ccflags-y

The variable $(PWD) is useless, and it may break the compilation.
For example, it breaks the kernel compilation when it's done with
buildroot :

  /home/trem/Codes/armadeus/armadeus/buildroot/output/host/usr/bin/ccache
/home/trem/Codes/armadeus/armadeus/buildroot/output/host/usr/bin/arm-buildroot-linux-uclibcgnueabi-gcc
-Wp,-MD,drivers/usb/gadget/legacy/.hid.o.d  -nostdinc -isystem
/home/trem/Codes/armadeus/armadeus/buildroot/output/host/usr/lib/gcc/arm-buildroot-linux-uclibcgnueabi/4.7.3/include
-I./arch/arm/include -Iarch/arm/include/generated  -Iinclude
-I./arch/arm/include/uapi -Iarch/arm/include/generated/uapi
-I./include/uapi -Iinclude/generated/uapi -include
./include/linux/kconfig.h -D__KERNEL__ -mlittle-endian -Wall -Wundef
-Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common
-Werror-implicit-function-declaration -Wno-format-security
-fno-dwarf2-cfi-asm -mabi=aapcs-linux -mno-thumb-interwork -mfpu=vfp
-funwind-tables -marm -D__LINUX_ARM_ARCH__=5 -march=armv5te
-mtune=arm9tdmi -msoft-float -Uarm -fno-delete-null-pointer-checks -O2
--param=allow-store-data-races=0 -Wframe-larger-than=1024
-fno-stack-protector -Wno-unused-but-set-variable -fomit-frame-pointer
-fno-var-tracking-assignments -g -Wdeclaration-after-statement
-Wno-pointer-sign -fno-strict-overflow -fconserve-stack
-Werror=implicit-int -Werror=strict-prototypes -DCC_HAVE_ASM_GOTO
-I/home/trem/Codes/armadeus/armadeus/buildroot/drivers/usb/gadget/
-I/home/trem/Codes/armadeus/armadeus/buildroot/drivers/usb/gadget/udc/
-I/home/trem/Codes/armadeus/armadeus/buildroot/drivers/usb/gadget/function/
-DMODULE  -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(hid)"
-D"KBUILD_MODNAME=KBUILD_STR(g_hid)" -c -o
drivers/usb/gadget/legacy/hid.o drivers/usb/gadget/legacy/hid.c
drivers/usb/gadget/epautoconf.c:23:26: erreur fatale: gadget_chips.h :
Aucun fichier ou dossier de ce type

This compilation line include :
..../buildroot/driver/usb/gadget
but the real path is :
..../buildroot/output/build/linux-3.17-rc1/driver/usb/gadget

Signed-off-by: Philippe Reynes <tremyfr@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>