mfd: 88pm860x-core: Convert to i2c_new_dummy_device

Move from i2c_new_dummy() to i2c_new_dummy_device(), so we now get an
ERRPTR which we use in error handling.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>