gpio: max732x: Fix possible NULL pointer dereference on i2c_new_dummy error

In max732x_probe() driver allocates dummy I2C device (if number of ports
is greater than 8) 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
dereferenced by i2c_smbus_read_byte() (called from max732x_readb()).

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>