HID: cougar: make compare_device_paths reusable

The function compare_device_paths from wacom_sys.c is generic
and useful for other drivers. Move the function to hid-core and
rename it as hid_compare_device_paths.

Signed-off-by: Daniel M. Lambea <dmlambea@gmail.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>