mac80211: refactor debugfs function generation code

refactor mac80211 debugfs code by using a format&copy function, instead of
duplicating the code for each generated function.

this change reduces about 600B from mac80211.ko

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>