Bluetooth: Fix trying to initiate connections when acting as LE slave

When we have at least one LE slave connection most (probably all)
controllers will refuse to initiate any new connections. To avoid
unnecessary failures simply check for this situation up-front and skip
the connection attempt.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>