ath10k: store firmware files in memory

Different FW versions may provide different
functions thus mean different hw capabilities
advertised to mac80211.

It is safe to swap firmware files on disk during
driver/device runtime without worries.

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