mfd: max77686: Fix regmap resource leak on driver remove

The regmap used by max77686 MFD driver was not freed with regmap_exit()
on driver exit. This lead to leak of resources.

Replace regmap_init_i2c() call in driver probe with initialization of
managed register map so the regmap will be properly freed by the device
management code.

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