greybus: interface: remove useless spinlock

Remove useless global interface spinlock that appeared to protect the
host-device interface list, but really did not as we are doing lock-less
look-ups by relying on the single-threaded SVC workqueue.

Document the locking assumptions.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>