HID: add sysfs speed attribute for wacom bluetooth tablet

The attribute allows to change reporting speed of tablet from userspace through
sysfs file. The attribute is RW, valid values: 0 is low speed, 1 is high speed.
High speed is the default setting. Using low speed is a workaround if you
experience lag when using the tablet.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>