net/dp8393x: disable user creation

Netcard needs an address space to write data to, which can't be specified
on command line.
This fixes a crash when user starts QEMU with "-device dp8393x"

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
