q800: fix mac_via RTC PRAM commands

q800: fix mac_via RTC PRAM commands

The command byte is not decoded correctly.

This patch reworks the RTC/PRAM interface and fixes the problem.
It adds a comment before the function to explain how are encoded commands
and some trace-events to ease debugging.

Bug: https://bugs.launchpad.net/qemu/+bug/1856549
Fixes: 6dca62a000 ("hw/m68k: add VIA support")
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20191219201439.84804-2-laurent@vivier.eu>