wl12xx: initialize rate_set on band rates initialization

In some corner cases, (invalid) 11g rates were used while
working on 11a band.

Take care of it by initializing rate_set according to the
configured band.

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