i2c_rtc: i2c_rtc_write(): Return values directly
i2c_rtc: i2c_rtc_write(): Restructure return logic
i2c_rtc: i2c_rtc_write(): Check datetime for validity
uart_protocol.c: Remove datetime validation

This is performed within i2c_rtc_write() implicitly.
uart_protocol.c: _time_set(): Remove assignment of variable
datetime.c: get_number_of_days_in_month(): Fix month addressing