iso-tp: revert CAN frame length check

[sshane]

Reverts #1315

The iso-tp spec allows optimization of the data field only with certain frame types. So while we know of no cases where this is a problem, we should revert until we handle properly

10.4.2.2	 CAN frame data optimization (TX_DL = 8)
If this solution is used, the DLC does not always need to be 8. If the N_PDU to be transmitted is shorter
than 8 bytes, then the sender may optimize the CAN bus load by shortening the CAN frame data to
contain only the number of bytes occupied by the N_PDU (no padding of unused data bytes). CAN frame
data optimization can only be used for an SF, FC frame or the last CF of a segmented message.