ath11k: refactor debugfs code into debugfs.c

If CONFIG_ATH11K_DEBUGFS is disabled there are warnings debug.c:

drivers/net/wireless/ath/ath11k/debug.c:36:20: warning: 'htt_bp_lmac_ring' defined but not used [-Wunused-variable]
   36 | static const char *htt_bp_lmac_ring[HTT_SW_LMAC_RING_IDX_MAX] = {
      |                    ^~~~~~~~~~~~~~~~
drivers/net/wireless/ath/ath11k/debug.c:15:20: warning: 'htt_bp_umac_ring' defined but not used [-Wunused-variable]
   15 | static const char *htt_bp_umac_ring[HTT_SW_UMAC_RING_IDX_MAX] = {
      |                    ^~~~~~~~~~~~~~~~

Fix this by refactoring debugfs code to debugfs.c. This also reduces the number
of ifdefs in debug.c and makes it easier to maintain the code.

No functional changes. Compile tested only.

Reported-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1600264523-12939-1-git-send-email-kvalo@codeaurora.org