HID: magicmouse: scroll on entire surface, not just middle of mouse

Previously, scroll events only occurred when the user moved a touch
along the middle of the touch surface. This is unintuitive for a normal
user who is not aware of this. The device has a uniform surface, so the
distinction is artificial. This change removes the touch area check for
a scroll event, which replicates the OS X behavior.

Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
Acked-by: Michael Poole <mdpoole@troilus.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>