mwifiex: key_material_v2 remove superfluous condition

We are using mac as source address in a memcpy.
In the lines below we can assume mac is not NULL.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>