cpu/stm32f0: optimizations to UART driver

Optimized the UART driver for less memory usage. See numbers for `tests/periph_uart_int`:

before:

```
   text	   data	    bss	    dec	    hex
  19464	    116	   5548	  25128	   6228
```

and after:

```
   text    data     bss     dec     hex
  18544     116    5548   24208    5e90
```