touchpad: Simplify tp_hysteresis

touchpad: Simplify tp_hysteresis

Once we get beyond the:

	if (abs(diff) <= margin)
		return center;

test, then diff is either > margin or < -margin, otherwise the test would
have triggered.

So the "return center + diff;" at the end will never be reached, and the
"else if (diff < -margin)" can be turned into a simple "else".

This commit does not just simplify tp_hysteresis, but (arguably more
important) also makes it clearer to the reader what it does.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>