Soundwire: add sdw_slave_get_scale_index helper

Currently, we only set peripheral frequency when the peripheral is
initialized. However, curr_dr_freq may change to get required bandwidth.
For example, curr_dr_freq may increase from 4.8MHz to 9.6MHz when the
4th stream is opened. Add a helper to get the scale index so that we can
get the scale index and program it.

Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20241218080155.102405-7-yung-chuan.liao@linux.intel.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>