rt2x00 : bail out of regbusy_read if device is removed.

platform rfkill is async thus we may try to read while the device is
already off.
Signed-off-by: Alban Browaeys <prahal@yahoo.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00pci.c

@@ -41,6 +41,9 @@ int rt2x00pci_regbusy_read(struct rt2x00_dev *rt2x00dev,
 {
 	unsigned int i;
 
+	if (!test_bit(DEVICE_STATE_PRESENT, &rt2x00dev->flags))
+		return 0;
+
 	for (i = 0; i < REGISTER_BUSY_COUNT; i++) {
 		rt2x00pci_register_read(rt2x00dev, offset, reg);
 		if (!rt2x00_get_field32(*reg, field))