wl12xx: add TX_HW_ATTR_HOST_ENCRYPT flag

In WEP shared authentication, we encrypt the auth frame
in the host, and we want the fw to pass it as-is.
Use the TX_HW_ATTR_HOST_ENCRYPT flag in order to indicate
it to the fw.

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