HID: logitech-dj/hidpp: Add info/warn/err messages about 27 MHz keyboard encryption

HID: logitech-dj/hidpp: Add info/warn/err messages about 27 MHz keyboard encryption

By default 27 MHz Logitech keyboards send the keypresses to the receiver
in plain text. Logitech's Windows "Keyboard and Mouse settings" tool allows
configuring encryption for the keyboard wireless link.

Now there also is a Linux tool for setting this up:
https://gitlab.freedesktop.org/jwrdegoede/logitech-27mhz-keyboard-encryption-setup

Once a keyboard is paired in encrypted mode, it will no longer work with
any receiver other then the receiver it was paired with.
If a user accidentally tries to pair another keyboard with the receiver
with which a keyboard has been paired in encrypted mode, then the receiver
looses the encryption key and the originally paired keyboard will no longer
work with either that receiver, or another receiver until the keyboard's
encryption key is rewritten by another encrypted mode pairing using the
tool. Or until it is reset by following a special reset procedure.

This commit adds 3 new log messages related to 27 MHz keyboard link
encryption to the Logitech dj / hidpp drivers:

1. An info level message when keyboard encryption is being used
2. A warning message when keyboard encryption is not used
3. An error message when the encryption key has been lost and the
   keyboard will not work because of this

Messages 2. and 3. contain a link to the userspace tool to setup the
encryption. The linked page also contains instructions to clear the key
(without needing the tool) to help a user recover from the keyboard not
working in case 3.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>