HID: uhid: forward raw output reports to user-space

Some drivers that use non-standard HID features require raw output reports
sent to the device. We now forward these requests directly to user-space
so the transport-level driver can correctly send it to the device or
handle it correspondingly.

There is no way to signal back whether the transmission was successful,
moreover, there might be lots of messages coming out from the driver
flushing the output-queue. However, there is currently no driver that
causes this so we are safe. If some drivers need to transmit lots of data
this way, we need a method to synchronize this and can implement another
UHID_OUTPUT_SYNC event.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
HID: uhid: implement feature requests

HID standard allows sending a feature request to the device which is
answered by an HID report. uhid implements this by sending a UHID_FEATURE
event to user-space which then must answer with UHID_FEATURE_ANSWER. If it
doesn't do this in a timely manner, the request is discarded silently.

We serialize the feature requests, that is, there is always only a single
active feature-request sent to user-space, other requests have to wait.
HIDP and USB-HID do it the same way.

Because we discard feature-requests silently, we must make sure to match
a response to the corresponding request. We use sequence-IDs for this so
user-space must copy the ID from the request into the answer.
Feature-answers are ignored if they do not contain the same ID as the
currently pending feature request.

Internally, we must make sure that feature-requests are synchronized with
UHID_DESTROY and close() events. We must not dead-lock when closing the
HID device, either, so we have to use separate locks.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>