net: dsa: qca8k: check correct variable in qca8k_phy_eth_command()

This is a copy and paste bug.  It was supposed to check "clear_skb"
instead of "write_skb".

Fixes: 2cd54856 ("net: dsa: qca8k: add support for phy read/write with mgmt Ethernet")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>