mac80211: Add function to get probe request template for current AP

Chipsets with hardware based connection monitoring need to autonomically
send directed probe-request frames to the AP (in the event of beacon loss,
for example.)

For the hardware to be able to do this, it requires a template for the frame
to transmit to the AP, filled in with the BSSID and SSID of the AP, but also
the supported rate IE's.

This patch adds a function to mac80211, which allows the hardware driver to
fetch this template after association, so it can be configured to the hardware.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>