net/wireless/ethtool.h: drop unnecessary include of linux/ethtool.h

Everything including this header includes net/cfg80211.h, which
includes linux/netdevice.h, which includes linux/ethtool.h already.  Why
slow-down the build, even a little bit?
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/wireless/ethtool.h

 #ifndef __CFG80211_ETHTOOL__
 #define __CFG80211_ETHTOOL__
 
-#include <linux/ethtool.h>
-
 extern const struct ethtool_ops cfg80211_ethtool_ops;
 
 #endif /* __CFG80211_ETHTOOL__ */