Input: msg2638 - only read linux,keycodes array if necessary

The linux,keycodes property is optional.
Fix the driver not probing when it's not specified.

Fixes: c18ef503 ("Input: msg2638 - add support for msg2138 key events")
Signed-off-by: Vincent Knecht <vincent.knecht@mailoo.org>
Link: https://lore.kernel.org/r/20221130210202.2069213-1-vincent.knecht@mailoo.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>