atm: firestream: fix interruptible_sleep_on race

interruptible_sleep_on is racy and going away. This replaces the one use
in the firestream driver with the appropriate wait_event_interruptible
variant.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Chas Williams <chas@cmf.nrl.navy.mil>
Cc: linux-atm-general@lists.sourceforge.net
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>