ath10k: fix arvif->wep_keys clearing

The variable should be cleared regardless of
whether there's a peer associated with the key or
not.

This fixes case when user first associates with 2
WEP keys and then disconnects and connects with 1
WEP key. This resulted in WEP key count being 2 in
the driver leading to default keyidx fixup
failure.

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