drivers: lora: fix RtcGetCalendarTime()

Uptime in milliseconds is assigned to uint32_t variable, which results
in integer overflow after enough time has expired. Additionally
milliseconds part (which should be 0-999) is assigned directly from
uptime, without subtracting full seconds.

Fix both issues by using int64_t variable and calculating milliseconds
with modulo.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>