Bluetooth: shell: make behaviour of iso cmd same as bt cmd

When no parameters are given to the 'bt' command it prints
the help message. The 'iso' command responds with 'missing
subcommand'. This commit updates the behaviour of the iso
command so that it behaves the same as the bt command, i.e.
prints a help message

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>