soundwire: bus: add sdw_slave_get_current_bank helper

soundwire: bus: add sdw_slave_get_current_bank helper

There has been 2 instances of this helper in codec drivers,
it does not make sense to keep duplicating this part of code.

Lets add a helper sdw_get_current_bank() for codec drivers to use it.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@oss.qualcomm.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://patch.msgid.link/20250909121954.225833-5-srinivas.kandagatla@oss.qualcomm.com
Signed-off-by: Mark Brown <broonie@kernel.org>