ACPI / hotplug / PCI: Drop handle field from struct acpiphp_func

The ACPI handle stored in struct acpiphp_func is also stored in the
struct acpiphp_context object containing it and it is trivial to get
from a struct acpiphp_func pointer to the handle field of the outer
struct acpiphp_context.

Hence, the handle field of struct acpiphp_func is redundant, so drop
it and provide a helper function, func_to_handle(), allowing it
users to get the ACPI handle for the given struct acpiphp_func
pointer.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>