MIPS: c-r4k: Fix GCC 4.6.0 build error

  CC      arch/mips/mm/c-r4k.o
arch/mips/mm/c-r4k.c: In function 'probe_scache':
arch/mips/mm/c-r4k.c:1078:6: error: variable 'tmp' set but not used [-Werror=unused-but-set-variable]
cc1: all warnings being treated as errors

Older GCC versions didn't warn about the unused variable tmp because it was
getting initialized.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>