mac80211: add NULL terminator to debugfs_netdev write buf

Some debugfs write functions call kstrto* functions, which
assume the string is null-terminated. Make it valid by changing
ieee80211_if_write() to use static buffer instead of allocating
one, and set the last char to NULL.

(The write functions try to parse some integer/mac address,
so 64 bytes buffer should be enough)

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