usb: gadget: f_fs: preserve wMaxPacketSize across usb_ep_autoconfig() call

usb_ep_autoconfig() treats the passed descriptor as if it were an fs
descriptor. In particular, for bulk endpoints, it clips wMaxPacketSize
to 64. This patch preserves the original value.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>