ath10k: make wmi_service_name() warn about missing service ids

When adding a new value to enum wmi_service it's very easy to miss that the new
value should be also added to wmi_service_name() mapping function. Modify the
function so that GCC can now warn about this:

drivers/net/wireless/ath/ath10k/wmi.h:385:2: warning: enumeration value 'WMI_SERVICE_FOO' not handled in switch [-Wswitch]

And also add a reminder to the enum.

Thanks to Jouni Malinen for the idea.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>