batman-adv: Fix use of seq_has_overflowed()

net-next commit 6d91147d ("batman-adv: Remove uses of return value
of seq_printf") incorrectly changed the overflow occurred return from
-1 to 1.  Change it back so that the test of batadv_write_buffer_text's
return value in batadv_gw_client_seq_print_text works properly.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>