ath10k: bump up FW API to 6

For QCA6174 hw3.0, since WLAN.RM.4.4-00022-QCARMSWPZ-2, it starts to
support the board ID information from otp, with some devices released on
the market that didn't calibrated with OTP, will have 0 for board ID
information, which cause the backward compatibility issue and was fixed
in commit 'd2e202c0 ("ath10k: ignore configuring the incorrect board_id")'

So bump the fw api version to differentiate the latest firmware support.

Signed-off-by: Ryan Hsu <ryanhsu@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>