libertas: Fix ethtool statistics

Fix various problems:
 - We converted MESH_ACCESS to a direct command but missed this caller.
 - We were trying to access mesh stats even on meshless firmware.
 - We should really zero the buffer if something goes wrong.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>