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>

Documentation/serial/driver

@@ -187,7 +187,6 @@ hardware.
 	ctl.
 
 	Locking: caller holds port->mutex
-	This call must not sleep
 
   startup(port)
 	Grab any interrupt resources and initialise any low level driver