usb: dwc3: dwc3-of-simple: Function headers are not good candidates for kerneldoc

usb: dwc3: dwc3-of-simple: Function headers are not good candidates for kerneldoc

Fixes the following kernel build warning(s):

 drivers/usb/dwc3/dwc3-of-simple.c:25: warning: cannot understand function prototype: 'struct dwc3_of_simple '

Cc: Felipe Balbi <balbi@kernel.org>
Cc: iivanov@mm-sol.com
Cc: Sundeep Bhatta <subbaraya.sundeep.bhatta@xilinx.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200702144625.2533530-19-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>