optimize default afe settings

backend/gt68xx_devices.c

@@ -1593,8 +1593,8 @@ static GT68xx_Model iriscan_express_2_model = {
   0,					/* CCD distcance for CCD with 6 lines) */
 
   COLOR_ORDER_BGR,		/* Order of the CCD/CIS colors */
-  {0x24, 0x0a, 0x23, 0x0f, 0x23, 0x0b},	/* Default offset/gain */
-  {0x100, 0x100, 0x100},	/* Default exposure parameters */
+  {0x22, 0x0c, 0x22, 0x0a, 0x24, 0x09},	/* Default offset/gain */
+  {0x157, 0x157, 0x157},	/* Default exposure parameters */
   SANE_FIX (2.0),			/* Default gamma value */
 
   SANE_TRUE,				/* Is this a CIS scanner? */