usb: gadget: udc: Fix duplicate uclass name

usb: gadget: udc: Fix duplicate uclass name

Currently both USB host uclass and USB gadget uclass are using the same
name "usb" which break uclass functions like uclass_get_by_name().

Rename the uclass to "usb_gadget" to fix, also makes bind/unbind by class
index (or sequence) working.

This breaks the capacity of using "usb" as DT alias sequence numbering
which needs a fix afterwards.

Signed-off-by: Zixun LI <admin@hifiphile.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/all/20240802092820.917450-1-admin@hifiphile.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>