mrf24j40: Fix en error handling path in 'mrf24j40_probe()'

If this check fails, we must release some resources as done everywhere
else in this function before returning an error code.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Alan Ott <alan@signal11.us>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>