wireless: clean up sysfs code using %pM

Remove converting the MAC address to a string by a direct byte
conversion and use %pM instead, since the code is now boilerplate
use a macro to define the show functions, and also use the shorter
__ATTR_RO macro to define the attributes.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>