net: mscc: ocelot: remove set but not used variable 'phy_mode'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/mscc/ocelot_board.c: In function 'mscc_ocelot_probe':
drivers/net/ethernet/mscc/ocelot_board.c:262:17: warning:
 variable 'phy_mode' set but not used [-Wunused-but-set-variable]
   enum phy_mode phy_mode;

It never used since introduction in
commit 71e32a20 ("net: mscc: ocelot: make use of SerDes PHYs for handling their configuration")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>