User space might want to test if driver supports testmode. Adding testmode
to the list of supported commands makes this easier.

I omitted testmode_dump() in purpose. I assume all drivers implementing
testmode_dump() will also implement testmode_cmd().
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>