can: janz-ican3: cleanup of ican3_to_can_frame and can_frame_to_ican3

This patch cleans up the ICAN3 to Linux CAN frame and vice versa
conversion functions:

- RX: Use get_can_dlc() to limit the dlc value.
- RX+TX: Don't copy the whole frame, only copy the amount of bytes
  specified in cf->can_dlc.

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