HID: surface-hid: Add support for legacy keyboard interface

Add support for the legacy keyboard (KBD/TC=0x08) HID transport layer of
the Surface System Aggregator Module (SSAM) to the Surface HID driver.
On Surface Laptops 1 and 2, this interface is used to connect the
integrated keyboard.

Note that this subsystem interface essentially provides a limited HID
transport layer. In contrast to the generic HID interface (TC=0x15) used
on newer Surface models, this interface only allows (as far as we know)
for a single device to be connected and is otherwise severely limited in
terms of support for feature- and output-reports. Specifically, only
caps-lock-LED output-reports and a single read-only feature-report are
supported.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

HID: Add support for Surface Aggregator Module HID transport

Add a HID transport driver to support integrated HID devices on newer
Microsoft Surface models (specifically 7th-generation, i.e. Surface
Laptop 3, Surface Book 3, and later).

On those models, the internal keyboard and touchpad (as well as some
other HID devices with currently unknown function) are connected via the
generic HID subsystem (TC=0x15) of the Surface System Aggregator Module
(SSAM). This subsystem provides a generic HID transport layer, support
for which is implemented by this driver.

Co-developed-by: Blaž Hrastnik <blaz@mxxn.io>
Signed-off-by: Blaž Hrastnik <blaz@mxxn.io>
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>