ieee802154: mrf24j40: fix incorrect mask in mrf24j40_stop

It seems that this is a copy/paste error and the proper bit masking is:
BIT_TXNIE | BIT_RXIE

This issue was detected with the help of Coccinelle.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Fixes: 7d840545 ("mrf24j40: replace magic numbers")
Acked-by: Alan Ott <alan@signal11.us>
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>