wl12xx: use 1 spare TX block instead of two

All the new firmware versions (>=6.1.3.50.58 for STA and >=6.2.0.0.47
for AP) use 1 spare TX block.  We still want to support older
firmwares that require 2 spare blocks, so added a quirk to handle the
difference.

Also implemented a generic way of setting quirks that depend on the
firmware revision.

Signed-off-by: Luciano Coelho <coelho@ti.com>