usb: gadget: composite: Allow bMaxPower=0 if self-powered

Currently the composite driver encodes the MaxPower field of
the configuration descriptor by reading the c->MaxPower of the
usb_configuration only if it is non-zero, otherwise it falls back
to using the value hard-coded in CONFIG_USB_GADGET_VBUS_DRAW.
However, there are cases when a configuration must explicitly set
bMaxPower to 0, particularly if its bmAttributes also has the
Self-Powered bit set, which is a valid combination.

This is specifically called out in the USB PD specification section
9.1, in which a PDUSB device "shall report zero in the bMaxPower
field after negotiating a mutually agreeable Contract", and also
verified by the USB Type-C Functional Test TD.4.10.2 Sink Power
Precedence Test.

The fix allows the c->MaxPower to be used for encoding the bMaxPower
even if it is 0, if the self-powered bit is also set.  An example
usage of this would be for a ConfigFS gadget to be dynamically
updated by userspace when the Type-C connection is determined to be
operating in Power Delivery mode.

Co-developed-by: Ronak Vijay Raheja <[email protected]>
Acked-by: Felipe Balbi <[email protected]>
Signed-off-by: Ronak Vijay Raheja <[email protected]>
Signed-off-by: Jack Pham <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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