Bluetooth: IAS: Fix alert level loop

The loop was supposed to set the alert level to the highest
value by any device. The loop, however, only made sense if
CONFIG_BT_MAX_CONN > 1.

It has been modified to start from [0] instead of [1] and
the initial condition was modified.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>