usb: gadget: udc: do not clear gadget driver.bus

Before the commit fc274c1e ("USB: gadget: Add a new bus for gadgets")
gadget driver.bus was unused. For whatever reason, many UDC drivers set
this field explicitly to NULL in udc_start(). With the newly added gadget
bus, doing this will crash the driver during the attach.

The problem was first reported, fixed and tested with OMAP UDC and g_ether.
Other drivers are changed based on code analysis only.

Fixes: fc274c1e

 ("USB: gadget: Add a new bus for gadgets")
Cc: stable <stable@kernel.org>
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20230201220125.GD2415@darkstar.musicnaut.iki.fi
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>