wl12xx: Fix CMD_TEST regression via netlink.

CMD_TEST via netlink API has been broken since
e29c3f59cfbc38c3b481a2694b08962da19c4664: cmd and acx interface rework.

The user of the interface sends the request in a buffer without the
wl12xx_command header but expects the response to have the wl12xx_command
header (with id and status).

This patch reverts the e29c3f5 commit for cmd.c:wl12xx_cmd_test
and implements the needed wrapper functionality in netlink.c. Now the
API of wl12xx_cmd_test and rest of wl12xx_cmd_* commands in cmd.c are
similar.

Signed-off-by: Ari Kauppi <Ext-Ari.Kauppi@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>