net: dsa: tag_8021q: use "err" consistently instead of "rc"

Some of the tag_8021q code has been taken out of sja1105, which uses
"rc" for its return code variables, whereas the DSA core uses "err".
Change tag_8021q for consistency.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>