Bluetooth: shell: Fix shell build with out-of-tree controller

Fix shell build errors when building with combined host and controller
but the selected controller is not the in-tree zephyr controller, i.e
CONFIG_BT_CTLR=y, CONFIG_BT_LL_SW_SPLIT=n and cmd_scanx and others
are not defined.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>