bluetooth: tester: btp: fixes ext adv selection

Disabling extended advertising, CONFIG_BT_EXT_ADV, fails to compile cause
references not guarded ext_adv variable when tester_gap_create_adv_instance
is called. Fixes by taking in use IS_ENABLED as preferred way of guarding
Kconfig option. It enables compiling guarded code to get warning/errors and
not to link if not used.

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