include: drivers: bluetooth: Use DT_ENUM_IDX_OR to get HCI bus from DT

Use DT_ENUM_IDX_OR instead of DT_STRING_UPPER_TOKEN_OR to get HCI bus
from DT, in order to avoid the expansion of UART/SPI/etc. macros when
they exist in vendor's SoC headers.

Fixes #93675

Signed-off-by: Ioannis Damigos <ioannis.damigos.uj@renesas.com>

fix