wl12xx: Use a dedicated fw for PLT

A special PLT firmware is used for calibration.

Add multiple fw support by introducing a new fw_type member,
representing the currently saved fw (the actual fw state
can be determined by wl->state).

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