fpga: dfl: fix bug in port reset handshake

When putting the port in reset, driver must wait for the soft reset
acknowledgment bit instead of the soft reset bit.

Fixes: 47c1b19c (fpga: dfl: afu: add port ops support)
Signed-off-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Acked-by: Wu Hao <hao.wu@intel.com>
Reviewed-by: Tom Rix <trix@redhat.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>