can: isotp: implement cleanups / improvements from review

As pointed out by Jakub Kicinski here:
http://lore.kernel.org/r/20201009175751.5c54097f@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com


this patch addresses the remarked issues:

- remove empty line in comment
- remove default=y for CAN_ISOTP in Kconfig
- make use of pr_notice_once()
- use GFP_ATOMIC instead of gfp_any() in soft hrtimer context

The version strings in the CAN subsystem are removed by a separate patch.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/r/20201012074354.25839-1-socketcan@hartkopp.net


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