Revise attach/detach resource cleanup

• Unconditionally call *_stop() if device is in the tree. This is to prevent callouts from happening after the device is gone. Checks for bus_child_present() should be added in the future to keep from touching potentially non-existent hardware in *_detach(). Found by iedowse@.
• Always check for and free miibus children, even if the device is not in the tree since some failure cases could have gotten here.
• Call ether_ifdetach() in the irq setup failure case
• ti(4), xl(4): move ifmedia_init() calls to the beginning of attach so that ifmedia_removeall() can be unconditionally called on detach. There is no way to detect whether ifmedia has been initialized without using a separate variable (as tl(4) does).
• Add comments to indicate assumptions of code path