Input: synaptics - use u8 instead of unsigned char

The rest of the kernel uses u8, u16, etc for data coming form hardware,
let's switch ti using u8 here as well.

Also turn pkt_type into an enum.

Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>