usb: gadget: net2280: Add support for PLX USB338X

This patch adds support for the PLX USB3380 and USB3382.

This driver is based on the driver from the manufacturer.

Since USB338X is register compatible with NET2280, I thought that it
would be better to include this hardware into net2280 driver.

Manufacturer's driver only supported the USB33X, did not follow the
Kernel Style and contain some trivial errors. This patch has tried to
address this issues.

This patch has only been tested on USB338x hardware, but the merge has
been done trying to not affect the behaviour of NET2280.

Tested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>