mrf24j40: apply the security-enabled bit on secured outbound frames

We set the TXNSECEN bit of register TXNCON to on when transmitting a
security-enabled frame, as described in section 3.12.2 of the MRF
datasheet.
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Alexandre Macabies <web+oss@zopieux.com>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: Alan Ott <alan@signal11.us>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/net/ieee802154/mrf24j40.c

@@ -61,6 +61,7 @@
 #define REG_TXBCON0	0x1A
 #define REG_TXNCON	0x1B  /* Transmit Normal FIFO Control */
 #define BIT_TXNTRIG	BIT(0)
+#define BIT_TXNSECEN	BIT(1)
 #define BIT_TXNACKREQ	BIT(2)
 
 #define REG_TXG1CON	0x1C
@@ -551,6 +552,9 @@ static void write_tx_buf_complete(void *context)
 	u8 val = BIT_TXNTRIG;
 	int ret;
 
+	if (ieee802154_is_secen(fc))
+		val |= BIT_TXNSECEN;
+
 	if (ieee802154_is_ackreq(fc))
 		val |= BIT_TXNACKREQ;