iw: handle VHT extended NSS

iw: handle VHT extended NSS

No change in "Supported Channel Width" output format when extended NSS
is not supported.

Sample output for a PHY with 1/2 NSS ratio:

VHT Capabilities (0xb39b79f2):
	Max MPDU length: 11454
	Supported Channel Width: 160Mhz (1/2 NSS) 80+80Mhz (1/2 NSS)
	[...]
VHT extended NSS: supported

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Link: https://lore.kernel.org/r/405f5ed2c434846f8ab2730b34288c5f649b8915.camel@freebox.fr
Signed-off-by: Johannes Berg <johannes.berg@intel.com>