wl12xx: add an spi glue struct to keep wl and device side-by-side

In order to fully abstract the bus, we need to save the device
structure *beside* wl1271, instead of inside it.

This will help re-structuring the driver so that we avoid the
duplicated code in the bus modules.

Signed-off-by: Felipe Balbi <balbi@ti.com>
[forward-ported and cleaned up and rephrased commit message]
Signed-off-by: Luciano Coelho <coelho@ti.com>