ACPI: Replace struct acpi_bus_ops with enum type

Notice that one member of struct acpi_bus_ops, acpi_op_add, is not
used anywhere any more and the relationship between its remaining
members, acpi_op_match and acpi_op_start, is such that it doesn't
make sense to set the latter without setting the former at the same
time.  Therefore, replace struct acpi_bus_ops with new a enum type,
enum acpi_bus_add_type, with three values, ACPI_BUS_ADD_BASIC,
ACPI_BUS_ADD_MATCH, ACPI_BUS_ADD_START, corresponding to
both acpi_op_match and acpi_op_start unset, acpi_op_match set and
acpi_op_start unset, and both acpi_op_match and acpi_op_start set,
respectively.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Toshi Kani <toshi.kani@hp.com>