MIPS: Malta: make a needlessly global integer variable static

The variable `mips_revision_corid' is needlessly defined global in
arch/mips/mti-malta/malta-init.c, and this patch makes it static.

Build-tested with malta_defconfig.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>