ath10k: rename the helper which is used for off-channel tx

Rename ath10k_mac_need_offchan_tx_work() to ath10k_mac_tx_frm_has_freq()
to make it more meaningful. This helper will be used in the future
change. No functionality change.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>