USB: sl811: add Kconfig option for ISOCHRONOUS mode

Some bluetooth dongles want ISO mode, and the limited support that the
sl811 offers today is sufficient.  So add a Kconfig option for people
to optionally get access to the partial functionality.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>