MIPS: RB532: Fix build of prom code.

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>