Add possibility to set the setband parameter

Add possibility to set the setband parameter

Commit faf9a8585d3e187074c7991e1cd7fb3792baf53e added mechanism for
selecting 2.4 or 5 GHz band for scan operation. However, no mechanism
for setting the setband value was added at that time. This commit adds a
new SET ctrl_iface parameter to allow the setband functionality to be
used. "SET setband <AUTO/5G/2G>" can be used to select all bands, 5 GHz
band only, or 2.4 GHz band only.

Signed-off-by: Jouni Malinen <j@w1.fi>