drivers: lora: fix RtcGetCalendarTime()
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 <63506c06cfbc47ace147db1702f6e751f5ac2132@grinn-global.com>

finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr