wifi: Add support for wlan driver argument property.

This is needed on drivers, that for example, require a mac parameter to be sent
to the driver.

Partly derived from change by bukington - Iaae1a2db6dc7ee07a78f1f2718bdafc73528c4b1

Change-Id: I16c9633f5aa462ac4453525350a887c551f5f857