selftests: can: test_raw_filter.sh: add support of physical interfaces

selftests: can: test_raw_filter.sh: add support of physical interfaces

Allow the user to specify a physical interface through the $CANIF
environment variable. Add a $BITRATE environment variable set with a
default value of 500000.

If $CANIF is omitted or if it starts with vcan (e.g. vcan1), the test
will use the virtual can interface type. Otherwise, it will assume
that the provided interface is a physical can interface.

For example:

  CANIF=can1 BITRATE=1000000 ./test_raw_filter.sh

will run set the can1 interface with a bitrate of one million and run
the tests on it.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>