mac80211: remove ieee80211_rx_bss_get

its not used where, while we directly obtain ieee80211_bss's
pointer in ibss.c by calling cfg80211_get_bss

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>