arm/allwinner-a10: replace snprintf() with g_strdup_printf()

arm/allwinner-a10: replace snprintf() with g_strdup_printf()

Also fixes a GCC 12.0.1 false-positive:
../hw/arm/allwinner-a10.c: In function ‘aw_a10_realize’:
../hw/arm/allwinner-a10.c:135:35: error: ‘%d’ directive writing between 1 and 11 bytes into a region of size 8 [-Werror=format-overflow=]
  135 |             sprintf(bus, "usb-bus.%d", i);
      |                                   ^~

Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Message-Id: <20220420132624.2439741[email protected]>