USB: serial: mos7840: fix non-atomic allocation in write path

commit 3b7c7e52 upstream.

There is an allocation with GFP_KERNEL flag in mos7840_write(),
while it may be called from interrupt context.

Follow-up for commit 19125283 ("USB: kobil_sct: fix non-atomic
allocation in write path")

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/serial/mos7840.c

@@ -1524,8 +1524,8 @@ static int mos7840_write(struct tty_struct *tty, struct usb_serial_port *port,
 	}
 
 	if (urb->transfer_buffer == NULL) {
-		urb->transfer_buffer =
-		    kmalloc(URB_TRANSFER_BUFFER_SIZE, GFP_KERNEL);
+		urb->transfer_buffer = kmalloc(URB_TRANSFER_BUFFER_SIZE,
+					       GFP_ATOMIC);
 
 		if (urb->transfer_buffer == NULL) {
 			dev_err(&port->dev, "%s no more kernel memory...\n",