usb: gadget: composite: Allow vbus_draw() to be called with 0mA

Similar to commit 9ffbd89c ("USB: gadget: composite: Allow
bMaxPower=0 if self-powered"), c->MaxPower may be allowed to be 0
when the gadget's configuration indicates it is self-powered. Thus
allow 0 to be passed directly to usb_gadget_vbus_draw() instead of
defaulting to CONFIG_USB_GADGET_VBUS_DRAW which is non-zero.

Change-Id: I4bf677a31a449c7763e2decad4860909f359dcb7
Signed-off-by: Jack Pham <jackp@codeaurora.org>