ath11k: change ath11k_core_fetch_board_data_api_n() to use ath11k_core_create_firmware_path()

Use the helper added in previous comment to create the full path, instead of doing it manually.

No functional changes. Compile tested only.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1592316055-24958-9-git-send-email-kvalo@codeaurora.org