wifi: shell: Fix default band value

The enum is mainly to print output of band, so, the default value is 0
which means 2.4GHz, which is not correct when using it to configure like
in connect.

Fix the default value to unknown i.e., no user preference. This way we
can use same enum for both set and get.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>