mwifiex: remove superfluous condition

for_each_property_of_node is only executed if the
property prop 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>