hw/misc/mmio_interface: Return after error_setg() to avoid crash

QEMU currently abort()s if the user tries to specify the mmio_interface
device without parameters:

x86_64-softmmu/qemu-system-x86_64 -nographic -device mmio_interface
qemu-system-x86_64: /home/thuth/devel/qemu/util/error.c:57: error_setv:
 Assertion `*errp == ((void *)0)' failed.
Aborted (core dumped)

This happens because the realize function is trying to set the errp
twice in this case. After setting an error, the realize function
should immediately return instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
