s390/3270: avoid endless I/O loop with disconnected 3270 terminals

If a 3270 terminal is disconnected while the tty view is active
the 3270 driver goes into an endless loop of failed I/O requests
until the terminal is connected again.

Add code to the raw3270 interrupt handler to check for unit checks
due to failed I/O requests and put the device to sleep with the
RAW3270_FLAGS_BUSY flag until a unsolicited device end interrupt
indicates that the device can be used again. while we are at it
simplify the 3270 irq handling and remove unnecessary code.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/con3270.c

@@ -400,7 +400,7 @@ con3270_deactivate(struct raw3270_view *view)
 	del_timer(&cp->timer);
 }
 
-static int
+static void
 con3270_irq(struct con3270 *cp, struct raw3270_request *rq, struct irb *irb)
 {
 	/* Handle ATTN. Schedule tasklet to read aid. */
@@ -418,7 +418,6 @@ con3270_irq(struct con3270 *cp, struct raw3270_request *rq, struct irb *irb)
 		cp->update_flags = CON_UPDATE_ALL;
 		con3270_set_timer(cp, 1);
 	}
-	return RAW3270_IO_DONE;
 }
 
 /* Console view to a 3270 device. */