mx6sabre: Do not enable UMS with SPL

Since commit ad8aae82 ("mx6sabre: Enable User Mass Storage") SPL target
does not boot anymore due to the increased spl image size.

Only enable USB Mass Storage for the non-SPL target.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Tom Rini <trini@konsulko.com>