shell: Fix command lookup for non-default modules

The code was completely broken wrt command lookups when the command is
given in the format "<module> <cmd> <args...>". It would only work if
the default module is not set, which is almost never the case (as most
apps set it explicitly).

Refactor the command handling by moving more logic up to shell_exec(),
so that get_cb() does a lookup for a single module

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>