ath10k: do not overwrite max_antenna_gain

Seems like we have an old bug, where we incidently overwrites
the max_antenna_gain we pass to firmware, with zero value.

End of all we are artifically reducing the output power.

This patch removes the excessive assignment on max_antenna_gain,
which is being provided by regulatory domain, and consequently
improves the tx power.

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>