[MIPS] rbtx4938: Fix warnings

linux/arch/mips/pci/fixup-tx4938.c:21:5: warning: symbol 'pci_get_irq' was not declared. Should it be static?
linux/arch/mips/pci/fixup-tx4938.c:76: warning: passing argument 1 of 'pci_get_irq' discards qualifiers from pointer target type

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>