rtc: Fix compiler warning in driver initialization.

[fjpanag]

Summary

The RTC driver causes the following warning:

timers/rtc.c: In function ‘rtc_initialize’:
timers/rtc.c:851:34: warning: ‘%d’ directive output may be truncated writing between 1 and 11 bytes into a region of size 8 [-Wformat-truncation=]
  851 |   snprintf(devpath, 16, "/dev/rtc%d", minor);
      |                                  ^~
timers/rtc.c:851:25: note: directive argument in the range [-2147483648, 999]
  851 |   snprintf(devpath, 16, "/dev/rtc%d", minor);
      |                         ^~~~~~~~~~~~
timers/rtc.c:851:3: note: ‘snprintf’ output between 10 and 20 bytes into a destination of size 16
  851 |   snprintf(devpath, 16, "/dev/rtc%d", minor);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This PR fixes this warning.
The devpath is slightly enlarged, to ensure that all possible numbers can be reliably printed.

Impact

Practically none.

The buffer is enlarged by just 4 bytes, but it is also located in the stack. So it shouldn't have any actual impact on any practical application.

Testing

Build test only.