sd/milkymist-memcard: Fix error API violation

sd/milkymist-memcard: Fix error API violation

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

milkymist_memcard_realize() is wrong that way: it passes &err to
qdev_prop_set_drive_err() and qdev_realize_and_unref().  Currently
harmless, because the latter uses it only as first argument of
error_propagate().

Making qdev_prop_set_drive_err() fail involves abuse of -global.
Leave handling that to qdev_prop_set_drive(), like we do elsewhere.

Cc: Michael Walle <michael@walle.cc>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200622094227.1271650-17-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>