USB: gadget: at91_udc minor fix (there is no number six)

Fix a small glitch noted by Yannick Cote.  There is no endpoint number
six, so if a (broken) host wrongly tried to change or read status of
that endpoint, the driver could access reserved register space.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Yannick Cote <yanick@yanos.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>