ar71xx: fix LED setup for TL-WDR4900 v2

ar71xx: fix LED setup for TL-WDR4900 v2

In ar71xx there is only one combined mach file for Archer C5/C7 and
TL-WDR4900 v2. This one uses the same LED struct for all devices,
defining "green" LEDs for them. However, WDR4900 uses blue front
LEDs, while only C5/C7 uses green ones. Despite, in base-files
WDR4900 is actually set up with "blue" for the mentioned LEDs.

Thus, this patch creates a separate LED struct for WDR4900, so the
LEDs can be set up correctly. Despite, the wlan5g LED is removed as
it is controlled by ath9k chip for WDR4900 (in contrast to C5/C7).

Note: While front LEDs are blue, USB LEDs (on the back) are green,
so colors are mixed intentionally for the WDR4900 v2.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>