OMAP3: beagle: Fix build warning in beagle.c

Fix build warning

beagle.c:532: warning: initialization from incompatible pointer type

Signed-off-by: Dirk Behme <dirk.behme@googlemail.com>
CC: Jason Kridner <jkridner@beagleboard.org>
CC: Koen Kooi <koen@dominion.thruhere.net>
CC: Joel A Fernandes <agnel.joel@gmail.com>
Cc: Greg Turner <gregturner@ti.com>
CC: Sandeep Paulraj <s-paulraj@ti.com>
Acked-by: Jason Kridner <jkridner@beagleboard.org>

board/ti/beagle/beagle.c

@@ -486,7 +486,7 @@ int ehci_hcd_init(void)
  * Returns - 	1 if button is held down
  *		0 if button is not held down
  */
-int do_userbutton (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
+int do_userbutton(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 {
 	int     button = 0;
 	int	gpio;