ath10k: support ethtool stats

Add support for reading firmware stats through the ethtool
API.  This may be easier for applications to manipulate
compared to parsing a text based debugfs file.

kvalo: remove unneeded ifdefs, call ath10k_debug_fw_stats_request() and added
simple error handling

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>