rt2x00: Fix access permissions on debugfs files

Although most rt2x00 debugfs files don't contain
information which could compromise network security,
it is better to set the access permissions to root only.

This will be required when HW crypto is implemented,
because it could be possible to read the HW key from
the registers.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>