fotg210-udc: Don't DMA more than the buffer can take

Before this, it wrote as much as available into the buffer, even if it
didn't fit.

Fixes: b84a8dee ("usb: gadget: add Faraday fotg210_udc driver")
Signed-off-by: Fabian Vogt <fabian@ritter-vogt.de>
Link: https://lore.kernel.org/r/20210324141115.9384-7-fabian@ritter-vogt.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>