ASoC: core: Add support for getting TX and RX channels

Based on which codec dai is selected, slimbus channel numbers
have to be queried when use case is being enabled.
Change adds support to get the channel numbers.

Change-Id: Iffeaf58b2fa9851efdb9afe4dea59d0fef94b713
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>