octeontx2-af: Fix error code in rvu_mbox_init()

The error code was intended to be -EINVAL here, but it was accidentally
changed to returning success.  Set the error code.

Fixes: e53ee4ac ("octeontx2-af: CN20k basic mbox operations and structures")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>