net: atm: clean up a range check

The code works fine but the problem is that check for negatives is a
no-op:

	if (arg < 0)
		i = 0;

The "i" value isn't used.  We immediately overwrite it with:

	i = array_index_nospec(arg, MAX_LEC_ITF);

The array_index_nospec() macro returns zero if "arg" is out of bounds so
this works, but the dead code is confusing and it doesn't look very
intentional.

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