ar9170usb: more minor fixes

This patch contains a few more mostly random fixes for the USB front-end.

1. handle irq command response, instead of printing it to the console.

2. remove fixed FIXME.
    (real fix: "ar9170usb: reset device on resume". )

3. some more one-liner.
	- get rid of a useless "return;"
	- add a few branch prediction hints in hot-paths
	etc.

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