ath10k: define ath10k_debug_start/_stop as static inline

Otherwise if CONFIG_ATH10K_DEBUGFS won't be set we will
end up with multiple definitions and compilation failure
in each place the header is included.

Signed-off-by: Bartosz Markowski <bartosz.markowski@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>