Input: usbtouchscreen - move process_pkt() into main device structure

Input: usbtouchscreen - move process_pkt() into main device structure

In preparation of splitting big usbtouch_dev_info table into separate
per-protocol structures and constifying them move process_pkt() from the
device info into main drvice structure and set it up in probe().
We can derive if we should use single- or multi-packet handling based
on presence of get_pkt_len() method.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240712051851.3463657-4-dmitry.torokhov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>