ath11k: search DT for qcom,ath11k-calibration-variant

Board Data File (BDF) is loaded upon driver boot-up procedure. The right
board data file is identified on IPQ6018 using bus, qmi-chip-id and
qmi-board-id.

The problem, however, can occur when the (default) board data file cannot
fulfill with the vendor requirements and it is necessary to use a different
board data file.

This problem was already solved on ath10k by adding a ",variant=.*" at the
end of the board name. The same functionality must also be provided for
ath11k.

The device tree requires an additional string to define the variant name

    wifi@c000000 {
        status = "okay";
        qcom,ath11k-calibration-variant = "Cigtech-WF-188";
    };

This would create the boarddata identifier for the board-2.bin search

 *  bus=ahb,qmi-chip-id=0,qmi-board-id=18,variant=Cigtech-WF-188

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201015131501.1939685-2-sven@narfation.org