iw: Add getting and setting of TXQ params for phy

iw: Add getting and setting of TXQ params for phy

This adds commands to get and set the per-phy TXQ parameters for drivers
that use the intermediate TXQs. These are the settings and statistics that
are also available through /sys/kernel/debug/ieee80211/phyX/aqm.

Sample output:

$ iw phy phy0 get txq
Packet limit:		8192 pkts
Memory limit:		4194304 bytes
Quantum:		300 bytes
Number of queues:	4096
Backlog:		12 pkts
Memory usage:		52224 bytes
Packet limit overflows:	0
Memory limit overflows:	0
Hash collisions:	1

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>