Input: ati_remote2 - fix type confusion in device attribute handlers

Previously, the channel_mask and mode_mask attributes were registered on
the parent usb_device via sysfs_create_group(). When commit b20d6bf8014b
("Input: ati-remote2 - use driver core to instantiate device
attributes") converted the driver to let the driver core manage them via
dev_groups in struct usb_driver, the attributes moved to the bound
usb_interface devices, but the show and store callbacks were not updated
accordingly.

As a result, the callbacks still invoked to_usb_device() on a device
pointer belonging to a struct usb_interface, producing an invalid
usb_device pointer.

Use to_usb_interface() directly in the attribute handlers to obtain the
interface and its driver data.

Fixes: b20d6bf8014b ("Input: ati-remote2 - use driver core to instantiate device attributes")
Assisted-by: DeepSeek-V4.1-Flash
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Link: https://patch.msgid.link/20261009040302.3087205-1-vulab@iscas.ac.cn
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>