wl12xx: define wl12xx_vif

Define a per-vif data struct.
This struct holds all the vif-specifc data, which is
currently being hold by the global wl struct.

Start by moving the basic_rate_set field into it.

NOTE: in order to make the patches a bit smaller, start by
using wl->vif in some functions, instead of changing all
the function prototypes at once. finally, wl->vif will be
removed altogether.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>