Fix tty layer sleep/locking problem (again) ... when this is
called through the network stack (PPP) sleeping isn't allowed.
There's some bugtraq ID for this.

From: Oliver Neukum <oliver@neukum.org>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>