ath11k: rename debug_htt_stats.[c|h] to debugfs_htt_stats.[c|h]

For consistency with debugfs.c rename debug_htt_stats files and functions to
debugfs_htt_stats.

No functional changes. Compile tested only.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1600264523-12939-3-git-send-email-kvalo@codeaurora.org