ath10k: set phymode to 11b when NO_OFDM flag set

phymode should use 11b only if NO_OFDM flags is set.
Hence check up channel flag for NO_OFDM and set to
11b.

Signed-off-by: Peter Oh <poh@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>