This patch continues where the previous commit:
	"carl9170: fix async command buffer leak"
left off.

Similar to carl9170_reboot/carl9170_powersave, the
carl9170_async_regwrite* macros would leak the
temporary command buffer, if __carl9170_exec_cmd
fails to upload the command to the device.
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>