mfd: max8998: Fix possible NULL pointer dereference on i2c_new_dummy error

During probe the driver allocates dummy I2C device for RTC with i2c_new_dummy() but it does not check the return value of this call.

In case of error (i2c_new_device(): memory allocation failure or I2C
address cannot be used) this function returns NULL which is later used
by i2c_unregister_device().

If i2c_new_dummy() fails for RTC device, fail also the probe for
main MFD driver.

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