ath10k: make TARGET_NUM_PEERS match WMI_MAX_KEY_INDEX.

This appears to fix the problem of running out of keys
in the firmware.  Possibly it only hides the problem,
however.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>