wl12xx: use 802.11 header location after relocation to frame start

When operating with TKIP encryption, the function wl1271_tx_fill_hdr()
relocates the 802.11 header to the start of the frame, and leaves room
for the security header.

Some functions in the Tx path rely on the location of the header,
namely, for purposes of roaming in STA mode and connecting new stations
in AP mode. Call these functions only after the header is relocated.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>