hw/sh_serial: Use TARGET_PRIxPHYS rather than %x for physaddr

Switch a format string from %x to TARGET_PRIxPHYS so that it will
continue to work even if target_phys_addr_t is changed
to 64 bits in the future.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>