[MIPS] PCI: Make the pcibios_max_latency variable static

The pcibios_max_latency variable is needlessly defined global, and this
patch makes it static.

Build-tested using malta_defconfig.

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