mac_{old|new}world: Reduce number of QOM casts

mac_{old|new}world: Reduce number of QOM casts

By storing the device pointers in a variable with the right type the
number of QOM casts can be reduced which also makes the code more
readable.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <f701e5ae03c806969561a7fd7523ef407db6dc89.1666957578.git.balaton@eik.bme.hu>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>