mfd: max8925: Fix possible NULL pointer dereference on i2c_new_dummy error

commit 96cf3dedc491d2f1f66cc26217f2b06b0c7b6797 upstream.

During probe the driver allocates dummy I2C devices for RTC and ADC
with i2c_new_dummy() but it does not check the return value of this
calls.

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 or ADC devices, fail also the probe
for main MFD driver.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>