usb: gadget: net2280: use swap() instead of open coding it

usb: gadget: net2280: use swap() instead of open coding it

Address the following coccicheck warning:
drivers/usb/gadget/udc/net2280.c:940:20-21: WARNING opportunity for swap().
drivers/usb/gadget/udc/net2280.c:944:25-26: WARNING opportunity for swap().

by using swap() for the swapping of variable values and drop the tmp
variables (`tmp` and `end`) that are not needed any more.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Link: https://lore.kernel.org/r/20220407100459.3605-1-guozhengkui@vivo.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>