USB: storage: adjust comment in Kconfig

Since commit 65934a9a

 ("Make USB storage depend on SCSI rather than selecting
it [try #6]") the comment at the top of drivers/usb/storage/Kconfig is
incorrect. Adjust it to the current situation.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>