HID: magicmouse: properly account for scroll movement in state

Before this change, sequential scroll events would take a variable
amount of movement due to incorrect accounting. This change ensures all
scroll movements require a deterministic touch movement for an action to
occur.

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