net: ieee802154: don't use devm_pinctrl_get_select_default() in probe

Since commit ab78029e (drivers/pinctrl: grab default handles from device
core), we can rely on device core for setting the default pins.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Acked-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>