net: ti: cpsw: enable 10Mbps link speed support in rgmii mode

According to TRMs the 10Mbps link speed is supported in RGMII only when
CPSW2G MAC SL is configured for External Control ("in band") mode
CPSW_SL_MACCTRL.EXT_EN(18) = 1.

Hence update cpsw_slave_update_link() to follow documentation.

[1] https://patchwork.kernel.org/patch/10285239/


Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>