[linuxinput] handle keys not known by libevdev (#13632)

[linuxinput] handle keys not known by libevdev (#13632)

* [linuxinput] handle keys not known by libevdev

Previously if libevdev could not resolve a numeric event code to a
symbolic name the name "null" was used.
This is useless for the user and may lead to duplicate-channel errors if
multiple unknown keys are encountered.

Instead use the numeric code itself as channel name if no symbolic code
could be determined.

Reported-in: https://community.openhab.org/t/linuxinput-binding-and-mouse-capture/122612/8
Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>
* [linuxinput] add channel description

Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>