msmcobalt: fix channel configuration for SLIMBUS

Channel count for SLIM_RX_6 port cannot be set
since get_port_idx() returns invalid port id which
resulting in invalid channel configuration for headset
usecase. Fix by adding SLIM_RX_6 case in get_port_idx().

Change-Id: Iadd3e995d044198c711f744c11b62cec2f7902c0
Signed-off-by: Shiv Maliyappanahalli <smaliyap@codeaurora.org>