can: flexcan: flexcan_chip_start(): fix erroneous flexcan_transceiver_enable() during bus-off recovery

If the CAN controller goes into bus off, the do_set_mode() callback with
CAN_MODE_START can be used to recover the controller, which then calls
flexcan_chip_start(). If configured, this is done automatically by the
framework or manually by the user.

In flexcan_chip_start() there is an explicit call to
flexcan_transceiver_enable(), which does a regulator_enable() on the
transceiver regulator. This results in a net usage counter increase, as there
is no corresponding flexcan_transceiver_disable() in the bus off code path.
This further leads to the transceiver stuck enabled, even if the CAN interface
is shut down.

To fix this problem the
flexcan_transceiver_enable()/flexcan_transceiver_disable() are moved out of
flexcan_chip_start()/flexcan_chip_stop() into flexcan_open()/flexcan_close().

Fixes: e955cead ("CAN: Add Flexcan CAN controller driver")
Link: https://lore.kernel.org/r/20201118150148.2664024-1-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>