soundwire: bus: Update kernel doc for no_pm functions

The kernel doc should really have been updated when the no_pm versions
of the sdw_write/read functions were exported in commits:

commit 167790ab ("soundwire: export sdw_write/read_no_pm functions")
commit 62dc9f3f ("soundwire: bus: export sdw_nwrite_no_pm and
                      sdw_nread_no_pm functions")

Add the missing kernel doc.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230322164948.566962-2-ckeepax@opensource.cirrus.com


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