net: stmmac: dwmac-sun8i: Remove unnecessary PHY power check

sun8i_dwmac_unpower_internal_phy already checks if the PHY is powered,
so there is no need to do it again here.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: David S. Miller <davem@davemloft.net>