Char: n_hdlc, allow RESTARTSYS retval of tty write

Acked-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>