fs: Allow to compile CMD_UBIFS when SPL is enabled

Currently there is no UBIFS support in SPL. But macro CONFIG_CMD_UBIFS is
defined also when building SPL. when both CMD_UBIFS and SPL are enabled
then build process throw compile error.

Fix check for ubifs code in fstypes[] to allow compiling CMD_UBIFS only in
proper U-Boot.

Signed-off-by: Pali Rohár <pali@kernel.org>