usb: gadget: lpc32xx_udc: Convert to DEFINE_SHOW_ATTRIBUTE

Use DEFINE_SHOW_ATTRIBUTE macro to simplify the code.

Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>