A couple small fixes for the imx6_sdma driver...

Attach after interrupt controllers, since the attach function tries to
set up an interrupt handler.

Check for the availability of the required firmware early in the attach
code (before allocating resources). If the firmware is not available, set
a static var to remember that, so that if the device is re-probed on later
passes it won't repeatedly try to attach and then complain again about
missing firmware.