net/dp8393x: do not use memory_region_init_rom_device with NULL

Replace memory_region_init_rom_device() with memory_region_init_ram() and
memory_region_set_readonly().
This fixes a guest-triggerable QEMU crash when guest tries to write to PROM.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
[leon.alrae@imgtec.com: shorten subject length]
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
