net: dsa: qca8k: check the correct variable in qca8k_set_mac_eee()

This code check "reg" but "ret" was intended so the error handling will
never trigger.

Fixes: 7c9896e3 ("net: dsa: qca8k: check return value of read functions correctly")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>