powerpc/eeh: Declare pci_ers_result_name() as static

Function pci_ers_result_name() is a static function, although not declared
as such. This was detected by sparse in the following warning

	arch/powerpc/kernel/eeh_driver.c:63:12: warning: symbol 'pci_ers_result_name' was not declared. Should it be static?

This patch simply declares the function a static.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>