fix keyboards/mxss/rgblight.h (#8388)

fix keyboards/mxss/rgblight.h (#8388)

Since #7773 caused a build error for `mxss:default`, I made similar changes to 'keyboards/mxss/rgblight.h' as #7773 did to 'quantum/rgblight.h'.

**This commit does not change the build result.**

Testing script
```shell
 # build on versions earlier than PR #7773
 git checkout 0.8.24
 echo master > /tmp/master_md5.txt

 make mxss:default:clean
 make mxss:default
 md5 mxss_default.hex >> /tmp/master_md5.txt

 # build on this commit
 git checkout fix-keyboards-mxss-rgblight.h
 echo fix-keyboards-mxss-rgblight.h > /tmp/branch_md5.txt

 make mxss:default:clean
 make mxss:default
 md5 mxss_default.hex >> /tmp/branch_md5.txt

 diff -u /tmp/master_md5.txt /tmp/branch_md5.txt
```

Test result:
```
--- /tmp/master_md5.txt 2020-03-12 05:51:39.000000000 +0900
+++ /tmp/branch_md5.txt 2020-03-12 05:51:49.000000000 +0900
@@ -1,2 +1,2 @@
-master
+fix-keyboards-mxss-rgblight.h
MD5 (mxss_default.hex) = 3034b2504d0c7fc6bd8bf1dffb6b8486
```