mfd: 88pm860x: Fix I2C device resource leak on regmap init fail

During probe the driver allocates dummy I2C device for companion chip
and then allocates a regmap for it. If regmap_init_i2c() fails then the
I2C driver (allocated with i2c_new_dummy()) is not freed and this
resource leaks.

Cc: stable@vger.kernel.org
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>