OMAP: Fix compile issue

Fixes this compile error:
board.c: In function 'do_switch_ecc':
board.c:339: error: 'cmd_tbl_t' has no member named 'help'
make[1]: *** [board.o] Error 1
make[1]: Leaving directory `/db/psp_git/users/a0756819/u-boot/cpu/arm_cortexa8/omap3'
make: *** [cpu/arm_cortexa8/omap3/libomap3.a] Error 2

This is due to the fact that current command uses long
help for the usage print even if the CONFIG_SYS_LONGHELP
is not enabled. (Thanks Jean-Christophe for explanation).
Signed-off-by: NSanjeev Premi <premi@ti.com>

cpu/arm_cortexa8/omap3/board.c

@@ -331,7 +331,7 @@ static int do_switch_ecc(cmd_tbl_t * cmdtp, int flag, int argc, char *argv[])
 	return 0;
 
 usage:
-	printf ("Usage: nandecc %s\n", cmdtp->help);
+	printf ("Usage: nandecc %s\n", cmdtp->usage);
 	return 1;
 }