ath11k: fix resource unavailability for htt stats after peer stats display

htt stats are not working after htt peer stats display
and also after htt peer stats reset. Trying to dump htt
stats shows "Resource temporarily unavailable".
This is because of "ar->debug.htt_stats.stats_req" member is being
consecutively used for all htt stats without being reset
during the previous usage. Hence assigning NULL to this member
after freeing the allocated memory fixes the issue.

console logs below:
# echo 9 >/sys/kernel/debug/ath11k/ipq8074/mac1/htt_stats_type
# cat /sys/kernel/debug/ath11k/ipq8074/mac1/htt_stats_type
9
# cat /sys/kernel/debug/ath11k/ipq8074/mac1/htt_stats
cat: can't open '/sys/kernel/debug/ath11k/ipq8074/mac1/htt_stats'
: Resource temporarily unavailable

Signed-off-by: Sowmiya Sree Elavalagan <ssreeela@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1588592755-10427-1-git-send-email-ssreeela@codeaurora.org