USB: BKL removal: rio500

This driver had used BKL to guard against disconnect but
was incorrectly converted leaving an SMP race.
BKL was added to disconnect() to fix this race
BKL was removed from ioctl() as the mutex is sufficient
on its own.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>