ath10k: do not overwrite max_antenna_gain

Seems like there's an old bug where we overwrite the
antenna_gain with zero value.

This patch removes the excessive assignment of max_antenna_gain,
which should be provided by regulatory domain settings.

Signed-off-by: Marek Puzyniak <marek.puzyniak@tieto.com>
Signed-off-by: Bartosz Markowski <bartosz.markowski@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>