mx6sabre_common: Allow building CONFIG_USB_FUNCTION_MASS_STORAGE

Selecting CONFIG_USB_FUNCTION_MASS_STORAGE=y in the defconfig leads to
the following error:

cmd/built-in.o: In function `do_usb_mass_storage':
cmd/usb_mass_storage.c:217: undefined reference to `fsg_main_thread'
cmd/usb_mass_storage.c:170: undefined reference to `fsg_init'

CONFIG_USB_FUNCTION_MASS_STORAGE option is built only for
the non-SPL case as per the logic in drivers/usb/gadget/Makefile,
so remove the incorrect CONFIG_SPL ifdefery to fix the build
failure.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Shawn Guo <shawn.guo@linaro.org>

include/configs/mx6sabre_common.h

@@ -208,8 +208,7 @@
 #define CONFIG_IMX_HDMI
 #define CONFIG_IMX_VIDEO_SKIP
 
-#ifndef CONFIG_SPL
 #define CONFIG_USBD_HS
-#endif
+#define CONFIG_USB_FUNCTION_MASS_STORAGE
 
 #endif                         /* __MX6QSABRE_COMMON_CONFIG_H */