can: kvaser_pciefd: Change name of return code variable

Replace the variable name err used for return codes with the more
generic name ret. An upcoming patch series for adding MSI interrupts
will introduce code which also returns values other than return codes.
Renaming the variable to ret enables using it for both purposes.
This is applied to the whole file to make it consistent.

Signed-off-by: Martin Jocic <martin.jocic@kvaser.com>
Link: https://lore.kernel.org/all/20240614151524.2718287-8-martin.jocic@kvaser.com


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