nwz-370/380: increase adc button error margin

For some reason those targets have quite imprecise button voltages and the
old margin was too small. This should fix the button-not-working issue,
especially when the player is very hot and cold.

Change-Id: I9fcddd7f079cd1c4ee121567fb21a4a0cbc0562b

firmware/target/arm/imx233/sony-nwz/button-target.h

@@ -32,6 +32,8 @@ bool button_debug_screen(void);
 #define HAS_BUTTON_HOLD
 #endif
 
+#define IMX233_BUTTON_LRADC_MARGIN  100
+
 /* Main unit's buttons */
 #define BUTTON_POWER                0x00000001
 #define BUTTON_BACK                 0x00000002