net: dsa: xrs700x: fix unused warning for of_device_id

Fix unused variable warning that occurs when CONFIG_OF isn't defined by
adding __maybe_unused.

>> drivers/net/dsa/xrs700x/xrs700x_i2c.c:127:34: warning: unused
variable 'xrs700x_i2c_dt_ids' [-Wunused-const-variable]
   static const struct of_device_id xrs700x_i2c_dt_ids[] = {

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>