mrf24j40: use regmap for register access

This patch uses the regmap functions for transceiver register settings
where it's possible. This means everything except the hotpaths like
receive/transmit handling.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>