clk: rockchip: fix the rv1108 clk_mac sel register description

The source clock ordering is wrong, as shown in the TRM:
cru_sel24_con[8]
rmii_extclk_sel
clock source select control register
1'b0: from internal PLL
1'b1: from external IO

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Reviewed-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>