drivers: can: rename struct can_driver_config fields

Rename the "bus_speed" and "bus_speed_data" fields of struct
can_driver_config to "bitrate" and "bitrate_data" to match the
corresponding devicetree properties and the terminology used in the rest of
the CAN subsystem API.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>