ath79: ZTE MF281: use specific board definition file for qca9888

ath79: ZTE MF281: use specific board definition file for qca9888

Using board definition file extracted from stock firmware yields 50%
throughput improvement in RX direction under iperf3 test.
Make the device use temporary files from firmware_qca-wireless.git
temporarily, as well as select the specific variant in the device tree
files. The device uses same board file as the MF286C.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/17620
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>