arm64: dts: apq8016-sbc: Correct WLAN LED default-trigger

The TX status trigger of the wlan interface is named phy0tx, so this
updates the default-trigger for the WLAN LED to use that instead.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>