Check if the interface is indeed connected to a MAC before trying to
close the DPMAC object representing it. Without this check we end up
working with a NULL pointer.

Fixes: d87e6063 ("dpaa2-mac: export MAC counters even when in TYPE_FIXED")
Signed-off-by: NIoana Ciornei <ioana.ciornei@nxp.com>
Link: https://lore.kernel.org/r/20210111171802.1826324-1-ciorneiioana@gmail.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>