drm/amd/display: fix signedness bug in execute_synaptics_rc_command()

The "ret" variable needs to be signed for the error handling to work.

Fixes: 2ca97adccd ("drm/amd/display: Add Synaptics Fifo Reset Workaround")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c

@@ -569,7 +569,7 @@ static bool execute_synaptics_rc_command(struct drm_dp_aux *aux,
 	unsigned char rc_cmd = 0;
 	unsigned char rc_result = 0xFF;
 	unsigned char i = 0;
-	uint8_t ret = 0;
+	int ret;
 
 	if (is_write_cmd) {
 		// write rc data