usbtouchscreen: fix data reading for ITM touchscreens

ITM devices seem to report only garbage when not touched. update usbtouchscreen
to do data reading like itmtouch. also fix wrong mask on pressure bits.

Signed-off-by: Daniel Ritz <daniel.ritz@gmx.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>