m68k: fpu: frestore with NULL state should reset FPU state
Description of problem:
According to the PRM:

```
Floating-Point Status Register: Cleared if the state size is NULL; otherwise, not affected. 
```

But this does not currently happen.
Steps to reproduce:
1. set a value in fpsr
2. do frestore with state size zero
3. read back fpsr and notice it isn't zero.