bluetooth: tester: btp: refactors periodic adv selection

Takes in use IS_ENABLED as preferred way of guarding Kconfig option for
enabling periodic advertising api, CONFIG_BT_PER_ADV. It enables compiling
guarded code get warning/errors and not link it if not used.

Signed-off-by: Evgenii Kosenko <Evgenii.Kosenko@silabs.com>