usb gadget: Rename husb2dev -> usba

husb2dev was the internal name of the USB Device Controller on
AT32AP7000. Rename it to "atmel_usba", which is closer to the official
name used in documentation and marketing material.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>