ax25: cleanup a range test

The current test works fine in practice.  The "amount" variable is
actually used as a boolean so negative values or any non-zero values
count as "true".  However since we don't allow numbers greater than one,
let's not allow negative numbers either.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>