ath10k: support up to 64 vdevs

The (1 << x) - 1 trick won't work when you
are trying to fill up all 64 bits, so add special
case for that.

Signed-off-by: Ben Greear <greearb@candelatech.com>
[kvalo@qca.qualcomm.com: remove the sentence about moving limits]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>