rt2x00: Add TX/RX frame dumping facility

This adds TX/RX frame dumping capabilities through debugfs.
The intention is that with this approach debugging of rt2x00 is
simplified since _all_ frames going in and out of the device
are send to debugfs as well along with additional information
like the hardware descriptor.

Based on the patch by Mattias Nissler.
Mattias also has some tools that will make the dumped frames
available to wireshark: http://www-user.rhrk.uni-kl.de/~nissler/rt2x00/

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