fpga: xilinx-pr-decoupler: Use readl wrapper instead of pure readl

Driver has IO wrappers but xlnx_pr_decouple_read() is not used and readl is
used instead which is just wrong.
It is also generating sparse issue that xlnx_pr_decouple_read() is unused.
Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Tom Rix <trix@redhat.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/0381e4e8061c2fee182a104768e84feff3a82d25.1679412800.git.michal.simek@amd.comSigned-off-by: Xu Yilun <yilun.xu@intel.com>

drivers/fpga/xilinx-pr-decoupler.c

@@ -69,7 +69,7 @@ static int xlnx_pr_decoupler_enable_show(struct fpga_bridge *bridge)
 	if (err)
 		return err;
 
-	status = readl(priv->io_base);
+	status = xlnx_pr_decouple_read(priv, CTRL_OFFSET);
 
 	clk_disable(priv->clk);