usb: gadget: pxa25x_udc: document endianess better

When I wrote the cleanup patch series, it was not clear how
exactly big-endian mode works on ixp4xx, and whether the driver
was doing this correctly. After discussing with Krzysztof Hałasa,
this has been clarified, so I can update the comment let pxa25x
big-endian (which we don't support) work the same way as ixp4xx.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <balbi@kernel.org>