carl9170: fix usb pm suspend->resume woes

This patch revamps some common code-paths which are
shared between (re-)initialization and suspend/resume
subroutines. It also adds some helpful comments
about quirks and associated difficulties.

It's quite big, but it should fix #25382:
<https://bugzilla.kernel.org/show_bug.cgi?id=25382

>

And hopefully the code is robust enough to deal with
all possible suspend/resume scenarios without requiring
the user to do any sort of manual and possibly
dangerous work.

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