can: kvaser_pciefd: Refactor code

Refactor code;
 - Format code
 - Rename variables and macros
 - Remove intermediate variables
 - Add/remove blank lines
 - Reduce scope of variables
 - Add helper functions

Signed-off-by: Jimmy Assarsson <extja@kvaser.com>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/all/20230529134248.752036-14-extja@kvaser.com


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