ath10k: fix NULL dereference for injected packets

Tx processing functions dereference vif and caused
NULL to be dereferenced for injected frames.

Don't call these functions at all for injected
frames. It doesn't make much sense to do so
anyway.

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