net: phy: test the right variable in phy_write_mmd()

This is a copy and paste buglet.  We meant to test for ->write_mmd but
we test for ->read_mmd.

Fixes: 1ee6b9bc ("net: phy: make phy_(read|write)_mmd() generic MMD accessors")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>