Firmware "board" file name has changed from wil6210.board
to wil6210.brd by the FW generation tools.

Reflect this in the driver.
Signed-off-by: NVladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: NKalle Valo <kvalo@qca.qualcomm.com>