Bluetooth: Host: GAP Device Name write now add null char at the end

The function used to write the value of the GAP Device Name
characteristic now ensure that the string passed to `bt_set_name` is
null terminated.

Also fix a wrong offset calculation.

The function used to write the value of the GAP Device Name
characteristic was returning an error when the offset + the length of
data to write was superior **or equal** to the maximum size of the
device name.

This caused the actual maximum device name size to be reduced by 1 byte.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>