clk: rockchip: Initialize hw to error to avoid undefined behavior

We can get down to this return value from ERR_CAST() without
initializing hw. Set it to -ENOMEM so that we always return something
sane.

Fixes the following smatch warning:

drivers/clk/rockchip/clk-half-divider.c:228 rockchip_clk_register_halfdiv() error: uninitialized symbol 'hw'.
drivers/clk/rockchip/clk-half-divider.c:228 rockchip_clk_register_halfdiv() warn: passing zero to 'ERR_CAST'

Cc: Elaine Zhang <zhangqing@rock-chips.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Fixes: 956060a5 ("clk: rockchip: add support for half divider")
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>