can: gs_usb: gs_make_candev(): clean up error handling

Introduce a label to free the allocated candev in case of an error and
make use of if. Fix a memory leak if the extended bit timing cannot be
read. Extend the error messages to print the number of the failing
channel and the symbolic error name.

Link: https://lore.kernel.org/all/20220921193902.575416-4-mkl@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>