HID: magicmouse: add param for scroll speed

The new scroll_speed param takes an integer value from 0 to 63, where 0
is slowest and 63 is fastest. The default of 32 remains the same. This
parameter also affects scroll acceleration linearly.

A second part of this change is a tightly coupled modification to the
scroll acceleration. Previously, scroll acceleration could be reset
without lifting the scroll finger. This is rather unintuitive and hard
to control in the case where a user wants faster scrolling, but wants to
hold the scroll touch for longer than a moment.

Note that scroll acceleration levels are now 1-7, where 7 is slowest. In
the previous implementation, there were 8 levels defined, but it was
impossible to start at the slowest level. In order to keep the default
scroll speed unchanged, only 7 levels are used now.

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