ath11k: fix undefined reference to 'ath11k_debugfs_htt_ext_stats_handler'

kbuild bot reported than link fails when CONFIG_ATH11K_DEBUGFS is disabled:

drivers/net/wireless/ath/ath11k/dp_rx.c:1662: undefined reference to `ath11k_debugfs_htt_ext_stats_handler'

This was because I had missed to move the static inline version of the function
(which are used when CONFIG_ATH11K_DEBUGFS is disabled) to debufs_htt_stats.h.
Also move ath11k_debugfs_htt_stats_req() at the same time. And create a stub
also for ath11k_debugfs_htt_stats_init() for consistency, even if it's not
needed.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 568f0603

 ("ath11k: debugfs: move some function declarations to correct header files")
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1601024241-16594-1-git-send-email-kvalo@codeaurora.org