usb: class: cdc-wdm: Provide description for usb_cdc_wdm_register()'s manage_power arg

usb: class: cdc-wdm: Provide description for usb_cdc_wdm_register()'s manage_power arg

A good attempt was made to document everything else.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/class/cdc-wdm.c:961: warning: Function parameter or member 'manage_power' not described in 'usb_cdc_wdm_register'

Cc: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200715093209.3165641-6-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>