nubus: Use static functions where possible

This fixes a couple of warnings from 'make W=1':
drivers/nubus/nubus.c:790: warning: no previous prototype for 'nubus_probe_slot'
drivers/nubus/nubus.c:824: warning: no previous prototype for 'nubus_scan_bus'

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>