mfd: hi655x-pmic: Fix missing return value check for devm_regmap_init_mmio_clk

Since devm_regmap_init_mmio_clk can fail, add return value checking.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Chen Feng <puck.chen@hisilicon.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>