Bluetooth: Host: Add BT_CONN_INTERVAL_TO_US

The macro BT_CONN_INTERVAL_TO_MS was used a fair amount
of places, but it often was used with integers. This meant
that sometimes the resulting (integer) value would be
incorrect, as something like 7.5ms interval would not
be properly stored as a integer in millisecond units.

Adding BT_CONN_INTERVAL_TO_US allows users to still use
integers to store the result, but in a more accurate unit.

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