wil6210: update FW file name

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: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>