net: dsa: microchip: ksz9477: remove MII_CTRL1000 check from ksz9477_w_phy()

The reason why PHYlib may access MII_CTRL1000 on the chip without GBit
support is only if chip provides wrong information about extended caps
register. This issue is now handled by ksz9477_r_phy_quirks()

With proper regmap_ranges provided for all chips we will be able to
catch this kind of bugs any way. So, remove this sanity check.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>