can: janz-ican3: drop invalid skbs

The commit which added the janz-ican3 driver and commit
3ccd4c61

 "can: Unify droping of invalid tx skbs and netdev stats" were
committed into mainline Linux during the same merge window.

Therefore, the addition of this code to the janz-ican3 driver was
forgotten. This patch adds the expected code.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>