epicardium/rtc: fix numerically unstable subsecond decoding

the subsecond encoding function from epic_rtc_set_milliseconds and the
corresponding decoding function from epic_rtc_get_milliseconds are not
numerically stable.

i.e., encoding 5 milliseconds to 20 subsecs and immediately afterwards decoding
that yields 4 milliseconds.

Adding a bias of 999 (0.24 milliseconds) to the decoding function makes it
numerically stable, while never decoding any subsecond value to more than 999
milliseconds.