disk: Correct the conditions for SPL

These filesystems may have different settings for SPL and TPL. Use the
correct Makefile variable to handle this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/disk/Makefile b/disk/Makefile
index b197692..5ca10c5 100644
--- a/disk/Makefile
+++ b/disk/Makefile
@@ -6,8 +6,8 @@
 #ccflags-y += -DET_DEBUG -DDEBUG
 
 obj-$(CONFIG_$(SPL_TPL_)PARTITIONS)  += part.o
-obj-$(CONFIG_$(SPL_)MAC_PARTITION)   += part_mac.o
-obj-$(CONFIG_$(SPL_)DOS_PARTITION)   += part_dos.o
-obj-$(CONFIG_$(SPL_)ISO_PARTITION)   += part_iso.o
-obj-$(CONFIG_$(SPL_)AMIGA_PARTITION) += part_amiga.o
-obj-$(CONFIG_$(SPL_)EFI_PARTITION)   += part_efi.o
+obj-$(CONFIG_$(SPL_TPL_)MAC_PARTITION)   += part_mac.o
+obj-$(CONFIG_$(SPL_TPL_)DOS_PARTITION)   += part_dos.o
+obj-$(CONFIG_$(SPL_TPL_)ISO_PARTITION)   += part_iso.o
+obj-$(CONFIG_$(SPL_TPL_)AMIGA_PARTITION) += part_amiga.o
+obj-$(CONFIG_$(SPL_TPL_)EFI_PARTITION)   += part_efi.o