Allow users to skip splash screen by setting display time to zero ms

In this part of 
[https://github.com/fossasia/badgemagic-firmware/blob/c299d578bb86da4f71df890448e627ad9afbe0c5/src/ngctrl.c#L137-L139](the source code) there is a check:
```c
	uint16_t ms = *((uint16_t *)val);
	if (ms < SPLASH_MIN_SPEED_T)
		return -2;
```
which i find user hostile. Users shouldn't be forced to see a splash screen and be unable to opt out of it.