USB: gadget: AT91: fix typo in atmel_usba_udc driver

compile fix for bug introduced by 969affff)

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>