Bluetooth: Shell: Fix the command bt adv-data

The command did no longer set the user specified data if the user
specified more than 1 AD field.

We were using the return value of ad_init() to set the number
of AD fields. This did not take into account the number of
existing data fields. By chance this usually returned 1.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>