PCI/ACPI: acpiphp: Identify more removable slots

According to section 6.3.6 of the ACPI spec, the presence of an _RMV
method that evaluates to 1 is sufficient to indicate that a slot is
removable without needing an eject method. This patch refactors the
ejectable slot detection code a little in order to flag these slots as
ejectable and register them. Acpihp then binds to the expresscard slot
on my HP test machine.

Acked-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>