MIPS: pci-rt2880: make pcibios_init() static

MIPS: pci-rt2880: make pcibios_init() static

This function is only used in the current source file. Mark it as
static to fix the compilation error:

arch/mips/pci/pci-rt2880.c:267:12: error: no previous prototype for 'pcibios_init' [-Werror=missing-prototypes]
  267 | int __init pcibios_init(void)
      |            ^~~~~~~~~~~~

Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>