mrf24j40: add writeable missing reg

This patch adds a missing reg for writeable stuff for regmap.

Cc: Alan Ott <alan@signal11.us>
Signed-off-by: NAlexander Aring <aar@pengutronix.de>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

drivers/net/ieee802154/mrf24j40.c

@@ -310,6 +310,7 @@ mrf24j40_short_reg_writeable(struct device *dev, unsigned int reg)
 	case REG_TRISGPIO:
 	case REG_GPIO:
 	case REG_RFCTL:
+	case REG_SECCR2:
 	case REG_SLPACK:
 	case REG_BBREG0:
 	case REG_BBREG1: