ath10k: fix num_legacy_stations tracking

If a station was reassociated, i.e. due to change
of supported rates update via sta_rc_update() the
num_legacy_stations would be (incorrectly) bumped
up leading to unbalanced usage of the var. This in
turn could lock rtscts protection up as enabled.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>