ath79: ZTE MF286[,A,R]: use GPIO19 as ath9k LED

ath79: ZTE MF286[,A,R]: use GPIO19 as ath9k LED

With the pinctrl configuration set properly by the previous commit, the
LED stays lit regardless of status of 2.4GHz radio, even if 5GHz radio
is disabled. Map GPIO19 as LED for ath9k, this way the LED will show
activity for both bands, as it is bound by logical AND with output of
ath10k-phy0 LED. This works well because during management traffic,
phy*tpt triggers typically cause LEDs to blink in unison.

Link: <https://github.com/openwrt/openwrt/pull/9941>
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>