qtnfmac: remove duplicated assignment to mac

mac is being assigned twice, remove redundant 2nd assignment.

Detected by CoverityScan, CID#1437554 ("Incorrect expression")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>