MIPS: RB532: Fix build of prom code.

  CC      arch/mips/rb532/prom.o
/home/ralf/src/linux/linux-mips/arch/mips/rb532/prom.c: In function ‘prom_setup_cmdline’:
/home/ralf/src/linux/linux-mips/arch/mips/rb532/prom.c:75:22: error: variable ‘prom_envp’ set but not used [-Werror=unused-but-set-variable]

This warning exists in gcc 4.6.0 and newer.  Kernels 2.6.40 and newer use
-Wunused-but-set-variable to suppress it.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/rb532/prom.c

@@ -72,12 +72,11 @@ void __init prom_setup_cmdline(void)
 	static char cmd_line[COMMAND_LINE_SIZE] __initdata;
 	char *cp, *board;
 	int prom_argc;
-	char **prom_argv, **prom_envp;
+	char **prom_argv;
 	int i;
 
 	prom_argc = fw_arg0;
 	prom_argv = (char **) fw_arg1;
-	prom_envp = (char **) fw_arg2;
 
 	cp = cmd_line;
 		/* Note: it is common that parameters start