shared/shell: don't allow completion fallback

shared/shell: don't allow completion fallback

When using bluetoothctl, if the user enters a quotation mark before
pressing tab, the completion handler falls back to the shell's handler
which ends up printing the current folder's file content.  This behavior
is never desired in bt_shell's menu.

This was verified by typing " then tab-tab before and after the change as
been made to confirm the desired behavior and verified that all other
argument completions (such as address completion) continues to function.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>