net: atm: fix update of position index in lec_seq_next

net: atm: fix update of position index in lec_seq_next

The position index in leq_seq_next is not updated when the next
entry is fetched an no more entries are available. This causes
seq_file to report the following error:

"seq_file: buggy .next function lec_seq_next [lec] did not update
 position index"

Fix this by always updating the position index.

[ Note: this is an ancient 2002 bug, the sha is from the
  tglx/history repo ]

Fixes 4aea2cbff417 ("[ATM]: Move lan seq_file ops to lec.c [1/3]")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20201027114925.21843-1-colin.king@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>