ath79: fix PQI Air Pen support

Use the correct dts filename. It has to be <soc>_<manufacturer>_<device>.

Signed-off-by: Mathias Kresin <dev@kresin.me>

diff --git a/target/linux/ath79/dts/ar9330_pqi-air-pen.dts b/target/linux/ath79/dts/ar9330_pqi_air-pen.dts
similarity index 100%
rename from target/linux/ath79/dts/ar9330_pqi-air-pen.dts
rename to target/linux/ath79/dts/ar9330_pqi_air-pen.dts