Remove unnecessary header room allocation during Tx.

clean up in code dealing with skb headroom during tx.
the sanity check on skb headroom is unnecessary for QCA_WIFI_2_0 in
wlan_hdd_softap_tx_rx.c and wlan_hdd_tx_rx.c.

Driver does not need to request extra headroom during netdev
registration by passing it in hard_header_len from
hdd_alloc_station_adapter().

CRs-FIXED: 582501
Bug: 24342935
Signed-off-by: Amarnath Hullur Subramanyam <amarnath@codeaurora.org>