ARM: OMAP2+: Fix board_mux section type conflict when OMAP_MUX is not set

Otherwise we can get:

arch/arm/mach-omap2/mux.h:249:31: error: board_mux causes a section type conflict
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/mux.h

@@ -246,7 +246,7 @@ static inline void omap_hwmod_mux(struct omap_hwmod_mux_info *hmux, u8 state)
 {
 }
 
-static struct omap_board_mux *board_mux __initdata __maybe_unused;
+static struct omap_board_mux *board_mux __maybe_unused;
 
 #endif