soundwire: stream: Fix test for DP prepare complete

soundwire: stream: Fix test for DP prepare complete

In sdw_prep_deprep_slave_ports(), after the wait_for_completion()
the DP prepare status register is read. If this indicates that the
port is now prepared, the code should continue with the port setup.
It is irrelevant whether the wait_for_completion() timed out if the
port is now ready.

The previous implementation would always fail if the
wait_for_completion() timed out, even if the port was reporting
successful prepare.

This patch also fixes a minor bug where the return from sdw_read()
was not checked for error - any error code with LSBits clear could
be misinterpreted as a successful port prepare.

Fixes: 79df15b7d37c ("soundwire: Add helpers for ports operations")
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210618144745.30629-1-rf@opensource.cirrus.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>