mac802154: fix kbuild test robot warning

This patch fixs the following kbuild test robot warning:

coccinelle warnings: (new ones prefixed by >>)

>> net/mac802154/cfg.c:53:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>