ath10k: allow the supported rate change by reassociate peer

IBSS mode requires the changing of supported rate. Do this
by reassociate the peer. The investigation shows that if move
from legacy to HT, the rate control won't work after changing
the supported rate. But once changing the supported rate to HT,
user can assign the TxRate in HT mode.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>