mrf24j40: fix Missing a blank line after declarations

Signed-off-by: Varka Bhadram <varkab@cdac.in>
Acked-by: Alan Ott <alan@signal11.us>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>