staging: gpib: Return -EINTR on device clear

When the ATN (Attention) line is asserted during a read we get a
NIUSB_ATN_STATE_ERROR during a read. For the controller to send a
device clear it asserts ATN. Normally this is an error but in the case
of a device clear it should be regarded as an interrupt.

Return -EINTR when the Device Clear Active State (DCAS) is entered
else signal an error with dev_dbg with status instead of just dev_err.

Signed-off-by: Dave Penkler <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/gpib/ni_usb/ni_usb_gpib.c

···
 		 */
 		break;
 	case NIUSB_ATN_STATE_ERROR:
-		retval = -EIO;
-		dev_err(&usb_dev->dev, "read when ATN set\n");
+		if (status.ibsta & DCAS) {
+			retval = -EINTR;
+		} else {
+			retval = -EIO;
+			dev_dbg(&usb_dev->dev, "read when ATN set stat: 0x%06x\n", status.ibsta);
+		}
 		break;
 	case NIUSB_ADDRESSING_ERROR:
 		retval = -EIO;