cfg80211: dynamically allocate per-tid stats for station info

With the addition of TXQ stats in the per-tid statistics the struct
station_info grew significantly. This resulted in stack size warnings
due to the structure itself being above the limit for the warnings.

Add an allocation function that those who want to provide per-tid
stats should use to allocate the tid array, i.e.
struct station_info::pertid.

Cc: Toke Høiland-Jørgensen <toke@toke.dk>
Fixes: 52539ca8 ("cfg80211: Expose TXQ stats and parameters to userspace")
Signed-off-by: Arend van Spriel <aspriel@gmail.com>
[johannes: fix missing BIT() and logic by removing]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>