fix "iw phy#0 info"

fix "iw phy#0 info"

Ben Greear reported that on a kernel that doesn't have the split
(and therefore filtered) wiphy dump feature, "iw phy#0 info" was
reporting data for all devices, rather than just one. This was a
result of using the split/filtered dump rather than a simple get
on "older" kernels (before 3.10!)

Fix this by checking the kernel feature before getting the data.