usb: gadget: configfs: change config attributes file operation

in order to add trace event in configfs function with same
struct gadget_info *gi parameter,
add struct config_usb_cfg *cfg variable in below functions,
gadget_config_desc_MaxPower_show(),
gadget_config_desc_MaxPower_store(),
gadget_config_desc_bmAttributes_show(),
gadget_config_desc_bmAttributes_store(),
this allow following patch easy change cfg to gi with helper function
cfg_to_gadget_info().

Signed-off-by: Linyu Yuan <quic_linyyuan@quicinc.com>
Link: https://lore.kernel.org/r/1634649997-28745-3-git-send-email-quic_linyyuan@quicinc.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>