ath11k: Fix TWT radio count

TWT feature fails on radio2 because physical device count is
hardcoded to 2. Set value dynamically.

Signed-off-by: Aloka Dixit <alokad@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200408174117.22957-1-alokad@codeaurora.org