USB: gadget: Kconfig: Change Max VBUS power usage default value

Currently the VBUS Max power usage is set to default value of 2.
Change this value to default of 500 to honour the code that uses
CONFIG_USB_GADGET_VBUS_DRAW to set the bMaxPower.

Change-Id: If27b422c23ad2a010744f9660cc0b376d02649df
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>