wifi: ath10k: drop chip-specific board data file name

The .fw.board parameter predates board-2.bin support. For all the
platforms, which define this parameter, it is equal to "board.bin".
Other platforms (like WCN3990) ommit it, limiting the ability to
provide board-specific data file.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240130-wcn3990-board-fw-v1-2-738f7c19a8c8@linaro.org