Bluetooth: df: fix ref antenna repeat after switch pattern exhausted

Direction finding functionality does antenna switching during CTE
reception in AoA mode and CTE transmission in AoD mode. Antennas are
switched according to user provided antenna switch pattern. If a CTE
length is enough to exhaust all antenna ids in a switch pattern then
Radio should loopback to reference antenna and continue from switching
from it.

Current implementation loops back to wrong antenna due to wrong index
used in GUARD_REF_ANTENNA_PATTERN_IDX. It was set to one instead of
zero. Zero is the index of reference antenna in Host provided antenna
switching pattern array.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>