Input: synaptics-rmi4 - fix enum_fmt

Do not enumerate all formats, some of which the device may not even
support. Instead, only report the one fixed format of the currently
selected input that will survive try_fmt/s_fmt.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>