USB: stop abuse of intfdata in cypress_m8

this driver uses usb_get_intfdata() == NULL as a test for disconnect().
You must not do that as this races with probe(). By the time you test
your erstwhile interface may already be somebody else's interface.
This fixes the close() method of cypress_m8 to use the recently introduced
flag and use locking against disconnect() where required in close().

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