wireless: a global static to local static improvement

There are two improvements in this simple patch:
1. wiphy_counter is a static var only used in one function, so
   can use local static instead of global static;
2. wiphy_counter wrap handling killed one comparision;

Signed-off-by: Denis ChengRq <crquan@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>