kconfig: BT: Default to using BT_CTLR when BT

As reported in issue #9494, BT samples are using the off-chip
UART-based BT controller on 52810. But the on-chip BT controller is
supported on 52810, so this is the reasonable default when BT is
enabled.

This patch enables BT_CTLR by default when BT is enabled.

This resolves #9494.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>