ieee802154_kw41z: Fix build error with SYS_LOG_INFO

The following fix:

commit c8b17ec4
Author: Tobias Aschenbrenner <tobias.aschenbrenner@blik.io>
Date:   Tue Dec 18 14:16:00 2018 +0100

    fix: kw41z: Use correct mapping for dBm

Was using SYS_LOG_INFO and should be using LOG_INFO.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>