phy: ti: am654-serdes: fix an use-after-free in serdes_am654_clk_register()

The regmap_node variable is still being used in the syscon_node_to_regmap()
call after the of_node_put() call, which may result in use-after-free.

Fixes: 71e2f5c5 ("phy: ti: Add a new SERDES driver for TI's AM654x SoC")
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Roger Quadros <rogerq@ti.com>
Reviewed-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>