USB: at91_udc: allow drivers that support high speed

This patch allows gadget drivers that support high speed (e.g. gadgetfs)
to work properly with at91_udc.

Fix suggested by Milan Svoboda in
http://marc.theaimsgroup.com/?l=linux-usb-devel&m=115822184711817

Signed-off-by: Wojtek Kaniewski <wojtekka@toxygen.net>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>