sd-boot: return 0 (not 1) from ticks_read() in fallback implementation

The single consumer of ticks_read() (i.e. time_usec()) checks for == 0
to detect the "not supported/invalid" case, hence actually return the
right value for that.

src/boot/efi/ticks.c

@@ -25,8 +25,7 @@ static UINT64 ticks_read(void) {
 }
 #else
 static UINT64 ticks_read(void) {
-        UINT64 val = 1;
-        return val;
+        return 0;
 }
 #endif