n_tty: Fix EOF push index when termios changes

Commit 40d5e090,
'n_tty: Fix EOF push handling' introduced a subtle state
change error wrt EOF push handling when the termios is
changed from non-canonical to canonical mode.

Reset line_start to the current read_tail index, not 0.

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