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>