RTC.setPeriodicCallback can't manage it 256 times a second

`RTC.setPeriodicCallback` works as advertised up to `Period::N128_TIMES_EVERY_SEC`, but `N256_TIMES_EVERY_SEC` gives the same callback rate as `N128_TIMES_EVERY_SEC.` Sample here shows the issue.

```
#include "RTC.h"

void setup() {
Serial.begin(9600);
while(!Serial)
; // do nothing
Serial.println("started");
RTC.begin();
RTCTime start = RTCTime(0UL);
RTC.setTime(start); // have to set a time, doesn't matter what
}

volatile unsigned long rate = 0;

void on_timer() {
static unsigned long count = 0;
static unsigned long last_secs = 0;
unsigned long secs = millis() / 1000;
if (secs != last_secs) {
rate = count;
count = 0;
last_secs = secs;
}
++count;
}

void run_test(Period period) {
if (!RTC.setPeriodicCallback(on_timer, period)) {
Serial.println("ERROR: periodic callback not set");
}
delay(1500); // ignore first result
for (unsigned int i = 0; i < 5; ++i) {
delay(1000);
Serial.print("rate = ");
Serial.println(rate);
}
}
void loop() {
Serial.println("Expect 64 per sec");
run_test(Period::N64_TIMES_EVERY_SEC);
Serial.println("Expect 128 per sec");
run_test(Period::N128_TIMES_EVERY_SEC);
Serial.println("Expect 256 per sec");
run_test(Period::N256_TIMES_EVERY_SEC);
}
```

Start with RTC.h and run the supplied sketch on the affected Arduino hardware to reproduce the callback rates for N64, N128, and N256. Trace the RTC.setPeriodicCallback entry point and its Period handling; done means N256_TIMES_EVERY_SEC produces the expected 256 callbacks per second without changing the existing rates.

Written by the indexing model from the issue text.