ath11k: debugfs: move some function declarations to correct header files

Some of the function declarations are for functions in debugfs_htt_stats.c and
debugfs_sta.c, move them to corresponding header files. As debugfs_sta.h didn't
exist create it.

Also in debugfs_htt_stats.h move dunction declarations to the end of the file.

No functional changes. Compile tested only.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1600264523-12939-4-git-send-email-kvalo@codeaurora.org