virnetdev: fix moving of 802.11 phys

virnetdev: fix moving of 802.11 phys

There was a couple of problems with the style fixes applied to the original
patch:

1.) virFileReadAllQuiet comparison was incorrectly parenthesized when moved
into a condition, causing the len to be set to the result of comparison. This,
together with the removed underflow check would underflow the phy buffer.

2.) The logic was broken. Failure to call "ip" would abort the function, thus
the "iw" branch would never be reached.

This aims to fix the issues and work around possible style complains :)

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
(cherry picked from commit 81b19ce46a9a65a00481403b8c0b15a8ac1367f2)