macfb: update macfb.c to use the Error API best practices

macfb: update macfb.c to use the Error API best practices

As per the current Error API best practices, change macfb_commom_realize() to return
a boolean indicating success to reduce errp boiler-plate handling code. Note that
memory_region_init_ram_nomigrate() is also updated to use &error_abort to indicate
a non-recoverable error, matching the behaviour recommended after similar
discussions on memory API failures for the recent nubus changes.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20211007221253.29024-3-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>