memstick: add "start" and "stop" methods to memstick device

In some cases it may be desirable to ensure that associated driver is not
going to access the media in some period of time.  "start" and "stop"
methods are provided therefore to allow it.

Signed-off-by: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>