wl1271: Flush TX buffers to air before going to idle

The mac80211 changes to idle almost immediately after transmitting some
frames, such as deauth etc. When going to idle, the wl1271 is disconnected,
which causes TX frames already on buffers, but not yet transmitted, to be
deleted.

To make sure deauth frames reach the air, allow the TX buffers to flush
before proceeding to idle.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>