Bluetooth: host: Define recommended per adv constants

These constants are based on (i.e. exactly as) the
recommended values for regular (i.e. non-periodic)
advertising.

The GAP spec does not actually specifiy these numbers (or any
numbers for periodic advertising), but they are sane
numbers to use for periodic advertisement.

The issue with using the non-periodic advertising
whereas the peridic advertising unit is 1.25ms.

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