can: dev: can_get_echo_skb(): factor out non sending code to __can_get_echo_skb()

commit a4310fa2f24687888ce80fdb0e88583561a23700 upstream.

This patch factors out all non sending parts of can_get_echo_skb() into
a seperate function __can_get_echo_skb(), so that it can be re-used in
an upcoming patch.

Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>