Correct SPL use of FS_EROFS

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_FS_EROFS defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Huang Jianan <jnhuang95@gmail.com>
diff --git a/fs/erofs/Makefile b/fs/erofs/Makefile
index 58af6a6..ef94d2d 100644
--- a/fs/erofs/Makefile
+++ b/fs/erofs/Makefile
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0+
 #
 
-obj-$(CONFIG_$(SPL_)FS_EROFS) = fs.o \
+obj-$(CONFIG_FS_EROFS) = fs.o \
 				super.o \
 				namei.o \
 				data.o \