ipw2200: prepare for const netdev->dev_addr

netdev->dev_addr will be come const soon, constify the argument
to command send to avoid compiler warnings.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211018235021.1279697-10-kuba@kernel.org