usb-storage: MAINTAINERS: Alan Stern is the new maintainer

At Matt Dharm's request, I am taking over maintainership of the
usb-storage driver.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>