can: softing: softing_card_shutdown(): add braces around empty body in an 'if' statement

This patch fixes the following warning when building the kernel with "W=1":

    warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

Link: http://lore.kernel.org/r/20201006203748.1750156-3-mkl@pengutronix.de
Fixes: 03fd3cf5

 ("can: add driver for Softing card")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>