staging: panel: fix interruptible_sleep_on race

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

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: devel@driverdev.osuosl.org
Cc: Willy Tarreau <willy@meta-x.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>