n_tty: Remove stale read lock comment

The stale comment refers to lock behavior which was eliminated in
commit 6d76bd26

,
n_tty: Make N_TTY ldisc receive path lockless.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>