soundwire: intel: introduce a helper to arm link synchronization

Move code from pre_bank_switch to dedicated helper, will be used in
follow-up patches as recommended by programming flows.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20200716150947.22119-4-yung-chuan.liao@linux.intel.com


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