wifi: shell: Allow setting band to 0 in connect

The help text for connect says

[-b, --band] 0: any band (2:2.4GHz, 5:5GHz, 6:6GHz]

but it disallows setting band to 0. Change this and allow
band 0 option when doing normal connect. Disallow setting
the band 0 when enabling AP mode.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>