usbtouchscreen: make ITM screens report BTN_TOUCH as zero when not touched

ITM screens send invalid x/y data when not touched. this was fixes a while ago
but the problem is if the screen is not touched anymore the driver never does
not report BTN_TOUCH as zero. fix it by sending the report with the last valid
coordinates when pressure is released.

Signed-off-by: Daniel Ritz <daniel.ritz@gmx.ch>
Cc: J.P. Delport <jpdelport@csir.co.za>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>