carl9170: fix async command buffer leak

If __carl9170_exec_cmd fails to upload an asynchronous
command to the device, the functions: carl9170_reboot
and carl9170_powersave will leak the temporary command
assembly buffer.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>