serial: doc: Use port->state instead of info

As of commit ebd2c8f6d2ec4012 ("serial: kill off uart_info"), the
circular transmission buffer is part of struct uart_state instead of
struct uart_info. Make it clear this structure is pointed to from struct
uart_port.

Change 'circ' to 'circ_buf' to match the structure name while we're at
it.

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/serial/driver

···
 
 	port->mctrl
 	port->icount
-	info->xmit.head (circ->head)
-	info->xmit.tail (circ->tail)
+	port->state->xmit.head (circ_buf->head)
+	port->state->xmit.tail (circ_buf->tail)
 
 The low level driver is free to use this lock to provide any additional
 locking.
···
 	RTS nor DTR; this will have already been done via a separate
 	call to set_mctrl.
 
-	Drivers must not access port->info once this call has completed.
+	Drivers must not access port->state once this call has completed.
 
 	This method will only be called when there are no more users of
 	this port.
···
 	Flush any write buffers, reset any DMA state and stop any
 	ongoing DMA transfers.
 
-	This will be called whenever the port->info->xmit circular
+	This will be called whenever the port->state->xmit circular
 	buffer is cleared.
 
 	Locking: port->lock taken.