isotp[send|recv]: change command line option for padding bytes

isotp[send|recv]: change command line option for padding bytes

Allow to specify the TX and RX padding content separately.
Due to the optional values it is possible to provide either TX or RX values
which enables and set the given values.

-p 55:AA => TX padding 0x55, RX padding 0xAA
-p 55    => TX padding 0x55
-p 55:   => TX padding 0x55
-p :AA   => RX padding 0xAA

Additionally the comment for the -P <mode> option was updated to point out the
checking mode of the padding in the received PDUs (rx padding).

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>