serial: doc: .break_ctl() may sleep

break_ctl() is not called from any sort of atomic context, so there is no
problem with it sleeping.

Reported-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>