Name the possible return values of palette fading functions

Also replace implicit conversion from gPalette.active into a status with
if statements, to make the return value clear.

I've also added comments when the check is redundant.

include/palette.h

@@ -9,6 +9,11 @@
 #define PLTT_BUFFER_SIZE 0x200
 #define PLTT_DECOMP_BUFFER_SIZE (PLTT_BUFFER_SIZE * 2)
 
+#define PALETTE_FADE_STATUS_DELAY 2
+#define PALETTE_FADE_STATUS_ACTIVE 1
+#define PALETTE_FADE_STATUS_DONE 0
+#define PALETTE_FADE_STATUS_LOADING -1
+
 enum
 {
     FAST_FADE_IN_FROM_WHITE,