iw: add support to set/get retry limit

Show the retry limit in phy details and also configure the
retry limit.

Signed-off-by: Ujjal Roy <royujjal@gmail.com>
iw: embrace the Developer Certificate of Origin for contributions

The Linux kernel started to embrace a light weight development contribution
agreement process called the Developer Certificate of Origin (DCO) circa
2004. Attorneys all over the planet have conferred a lot of respect and
appreciation for this document and it provides a light weight alternative to
cumbersome and controversial Contribution License Agreements (CLAs).

Similar to streamlining FOSS licenses we also all stand to gain from
streamlining a light weight contribution agreement process for development
and in light of this the the Linux Foundation recently has made the DCO a
standalone project [0] so that any FOSS project under any FOSS license can
take advantage of the same gains. The motivation and intent is documented
only currently on my blog post [1].

Lets take advantage of the new shiny DCO and embrace it for contributions.

NOTE: Contributions before this commit that still included a Signed-off-by
      line (presumably out of habit of typing -s with git commands) aren't
      necessarily covered by this agreement.

[0] http://developercertificate.org/
[1] http://www.do-not-panic.com/2014/02/developer-certificate-of-origin.html

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>