usb: renesas-xhci: Replace BIT(15) with macro

Replace BIT(15) with RENESAS_ROM_STATUS_ROM_EXISTS.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Link: https://lore.kernel.org/r/20210614215614.240489-1-mdf@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>