wl12xx: don't explicitly check for unjoined ibss

After the ibss carrier issue was fixed, we can revert
the following patch:

commit 48309fd4
Author: Shahar Lev <shahar@wizery.com>
Date:   Fri Oct 7 18:17:25 2011 +0200

    wl12xx: remove warning message during IBSS Tx

    mac80211 sets the carrier on an IBSS interface even when no network is
    joined. Ignore garbage frames transmitted on a disconnected IBSS
    interface without printing warnings.

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