ath10k: support CT firmware choices.

This allows one to select CT firmware for ath10k
if one prefers to try this firmware instead of stock
ath10k firmware.

The 10.1 (988X) firmware can actually be installed beside the
default firmware, but it will not be used as long as the firmware-5.bin
file exists.  Users could rename the files and reboot to use different
images.

The 99X0 (wave-2) firmware  uses the same firmware-5.bin name as default
firmware, so it cannot be installed at the same time as default firmware.

Signed-off-by: Ben Greear <greearb@candelatech.com>