mvsas: don't allow negative timeouts

There is a static checker warning here because "val" is controlled by
the user and we have a upper bound on it but allow negative numbers.
"val" appears to be a timeout in usec so this bug probably means we
have a longer timeout than we should.  Let's fix this by changing "val"
to unsigned.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jack Wang <jinpu.wang@profitbricks.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>