Added brace guards to macros with parameters to prevent unintended changed evaluation of the macro expression.
Minor code cleanups (remove redundant comments, fix spacing, etc.).
Added new USE_FLASH_DESCRIPTORS compile time option. By default, descriptors can now lie in mixed memory spaces (specified by a new parameter to the CALLBACK_USB_GetDescriptor() function) unless one of the USE_*_DESCRIPTORS compile time option is specified.