USB: enable USB-PERSIST by default

This patch (as1052) enables USB-PERSIST for all devices by default.
The user won't have to remember to enable it explicitly for devices
containing mounted filesystems.

Eventually userspace tools like hal may be able to set the persist
attribute automatically when a filesystem is mounted on a USB device.
When that time comes this patch can be reverted, if people think it
matters.

This approach has the advantage of giving the user the ability to turn
off USB-PERSIST for devices with mounted filesystems, rather than
making the kernel always assume it should be on.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>