usb: dwc3: ensure consistent types for dwc3_flush_cache

usb: dwc3: ensure consistent types for dwc3_flush_cache

The dwc3_flush_cache() call was declared and used inconsistently:
 * The declaration assumed 'int' for addresses (a potential issue
   when running in a LP64 memory model).
 * The invocation cast the address to 'long'.

This change ensures that both the declaration and usage of this
function consistently uses 'uintptr_t' for correct behaviour even
when the allocated buffers (to be flushed) reside outside of the
lower 32bits of memory.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>