HID: logitech-hidpp: remove hidpp_is_connected()

Remove the hidpp_is_connected() function wrapper, and have the callers
directly call hidpp_root_get_protocol_version() instead.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>