can: hi311x: fix a signedness bug in hi3110_cmd()

The hi3110_cmd() is supposed to return zero on success and negative
error codes on failure, but it was accidentally declared as a u8 when
it needs to be an int type.

Fixes: 57e83fb9 ("can: hi311x: Add Holt HI-311x CAN driver")
Link: https://lore.kernel.org/r/20210729141246.GA1267@kili


Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>