x25: '< 0' and '>= 0' test on unsigned

skb->len is an unsigned int, so the test in x25_rx_call_request() always
evaluates to true.

len in x25_sendmsg() is unsigned as well. so -ERRORS returned by x25_output()
are not noticed.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>