rtc: cmos: Make rtc_cmos sync offset correct

The offset for rtc_cmos must be -500ms to work correctly with the current
implementation of rtc_set_ntp_time() due to the following:

  tsched       twrite(t2.tv_sec - 1) 	 t2 (seconds increment)

twrite - tsched is the transport time for the write to hit the device,
which is negligible for this chip because it's accessed directly.

t2 - twrite = 500ms according to the datasheet.

But rtc_set_ntp_time() calculation of tsched is:

    tsched = t2 - 1sec - (t2 - twrite)

The default for the sync offset is 500ms which means that the write happens
at t2 - 1.5 seconds which is obviously off by a second for this device.

Make the offset -500ms so it works correct.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201206220541.830517160@linutronix.de