phy/micrel: Fix ID of KSZ9021

Right ID of KSZ9021 is 0x00221610.
Because lower 4bit is a revision number, it varies according to a chip.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Cc: David J. Choi <david.choi@micrel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>