Define and add .clang-format for //drivers/wifi/

This reflects the currenct practice as much as possible.
This can serve until Zircon/Fuchsia-wide style convergence is put in place.

See in action
https://fuchsia-review.googlesource.com/c/drivers/wifi/+/62789/

Change-Id: Ib9dbc591568d307bc2d1ef6d5d7c5651d135f74e