usb: atm: Replace in_interrupt() usage in comment

in_interrupt() is a pretty vague context description as it means: hard
interrupt, soft interrupt or bottom half disabled regions.

Replace the vague comment with a proper reasoning why spin_lock_irqsave()
needs to be used.
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Duncan Sands <duncan.sands@free.fr>
Cc: linux-usb@vger.kernel.org

Link: https://lore.kernel.org/r/20201019101110.944939915@linutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/atm/usbatm.c

@@ -249,7 +249,7 @@ static void usbatm_complete(struct urb *urb)
 	/* vdbg("%s: urb 0x%p, status %d, actual_length %d",
 	     __func__, urb, status, urb->actual_length); */
 
-	/* usually in_interrupt(), but not always */
+	/* Can be invoked from task context, protect against interrupts */
 	spin_lock_irqsave(&channel->lock, flags);
 
 	/* must add to the back when receiving; doesn't matter when sending */