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

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>