ath79: modify device name of I-O DATA WN-AC1600DGR2

ath79: modify device name of I-O DATA WN-AC1600DGR2

The hardware of I-O DATA WN-AC1600DGR3 has completely compatibility
with WN-AC1600DGR2, so modify the device name of WN-AC1600DGR2 to show
the images for DGR2 also support DGR3.

Specification of WN-AC1600DGR3:

- SoC        : Qualcomm Atheros QCA9557
- RAM        : DDR2 128 MiB
- Flash      : SPI-NOR 16 MiB
- WLAN       : 2.4/5 GHz
  - 2.4 GHz  : QCA9557 (SoC), 2T2R
  - 5 Ghz    : QCA9880, 3T3R
- Ethernet   : 5x 10/100/1000 Mbps
  - Switch   : QCA8337N
- LEDs/Input : 6x/6x (4x buttons, 1x slide-switch)
- UART       : through-hole on PCB
  - J1: Vcc, GND, TX, RX from ethernet port side
  - 115200n8

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>