sdvx: Allow setting digital amp level from sdvxio

For SDVX 1-4: passes amp values through as-is

For SDVX5: ignores the amp volume control (as the game uses windows volume controls, which will work without any hooks)

src/main/sdvxio/sdvxio.c

@@ -88,3 +88,8 @@ uint16_t sdvx_io_get_spinner_pos(uint8_t spinner_no)
 {
     return mapper_read_analog(spinner_no) * 4;
 }
+
+bool sdvx_io_set_amp_volume(uint8_t primary, uint8_t headphone, uint8_t subwoofer)
+{
+    return true;
+}

src/main/sdvxio/sdvxio.def

@@ -11,3 +11,4 @@ EXPORTS
     sdvx_io_set_loggers
     sdvx_io_set_pwm_light
     sdvx_io_write_output
+    sdvx_io_set_amp_volume