Bluetooth: Controller: Fix CIS and CIG teardown

Fix CIS and CIG teardown in Central and Peripheral Role.
cig->lll.num_cis value is used in peripheral role to count
active CISes, and cig->cis_count is used in central role.
Both central and peripheral will stop ticker for CIG when
num_cis and/or cis_count is zero, respectively, but only
peripheral role will release a CIG context. The Central
will keep the CIG context allocated and only mark it as
not started. Remove CIG command will be used for central
role to release a CIG context.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>