The usbhs_write32 function is not used outside of the rcar3.c
file, so fix the following sparse warning by making it static:

drivers/usb/renesas_usbhs/rcar3.c:26:6: warning: symbol 'usbhs_write32' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>