AMD get_bus_conf(): Drop bus_type array

Only ever used as lvalue, so no point creating the array.

Change-Id: I6699dfae9377a895e9bc4a52579d00ddcfa60a9f
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/6277
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Rudolf Marek <r.marek@assembler.cz>