tests: Don't use deprecated BT_CTLR Kconfig option

The new option is HAS_BT_CTLR, however since it's a promptless option,
disable the native link layer instead, which most likely was the intended
effect.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>

Author: jhedberg

tests/bluetooth/buf/prj.conf

@@ -2,7 +2,7 @@ CONFIG_TEST=y
 CONFIG_ZTEST=y
 
 CONFIG_BT=y
-CONFIG_BT_CTLR=n
+CONFIG_BT_LL_SW_SPLIT=n
 CONFIG_BT_H4=n
 
 # Needed to enable and test the iso rx pool