HID: handle cases of volume knobs generating relative values

There are some devices (for example Dell Multimedia Keyboard SK-8135) that have
a volume control knob which generates relative events instead of absolute.
hid-input maps them to ABS_VOLUME. HUT pages don't restrict volume to absolute
values.

Adding REL_VOLUME doesn't seem feasible, nothing knows how to handle it. This
patch translates relative ABS_VOLUME events into appropriate number of series
of VOLUME_UP or VOLUME_DOWN events respectively, so that userspace sees the
correct values in the end.

kernel.org bugzilla 5233

Reported-by: Jochen Eisinger <jochen@penguin-breeder.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>