clk: rockchip: don't return NULL when registering mmc branch fails

Avoid return NULL if rockchip_clk_register_mmc fails, otherwise
rockchip_clk_register_branches print "unknown clock type". The acutal
case is that it's a known clock type but we fail to regiser it, which
may makes user confuse the reason of failure.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>