usb: atm: Replace in_interrupt() usage in comment

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.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>