cpu/sam0_common: DAC: wait for DAC to be ready

The DAC can have some start-up delay.
If we try to write to it before it's ready, it will get stuck.

This happens now that `tests/driver_dac_dds` immediately sets a DAC
value after init.

The samd2x class of MCUs doesn't have this bit, but a quick test on
samd10 shows that it might not be nececary there - the DAC does not
get stuck when writing to it immediately after init.

cpu/sam0_common/periph/dac.c

@@ -128,6 +128,12 @@ int8_t dac_init(dac_t line)
     DAC->CTRLA.bit.ENABLE = 1;
     _sync();
 
+#ifdef DAC_STATUS_READY
+    /* wait for DAC startup */
+    const uint32_t mask = 1 << (DAC_STATUS_READY_Pos + line);
+    while (!(DAC->STATUS.reg & mask)) {}
+#endif
+
     return DAC_OK;
 }