Input: usbtouchscreen - correctly set 'phys'

This patch fixes a nasty typo in usbtouchscreen driver. The typo
is inherited from the original mtouchusb. It used to make the input
subsytem to incorrectly report the physical device ids to userspace
that in turn is very confusing for, e.g. XInput hotplug facilities
in setups with multiple identical touchscreens.

Signed-off-by: Vladimir Shebordaev <vshebordaev@mail.ru>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>