rsi: Correct RSI_NEEDED_HEADROOM in mac80211_attach.

Currently, RSI_NEEDED_HEADROOM is '80' for rsi driver, which is wrong.
As per rsi internal frame format, the RSI_NEEDED_HEADROOM shall be '84',
which is 64(dword_align) + 4(extended_desc) + 16(frame_desc).
Hence, corrected the needed headroom.

Signed-off-by: Amol Hanwate <amol.hanwate@redpinesignals.com>
Signed-off-by: Siva Rebbagondla <siva.rebbagondla@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>