mfd: 88pm860x: Fix possible NULL pointer dereference on i2c_new_dummy error

commit 159ce52a6b777fc82fa0b51c7440e25f9e4c6feb upstream.

During probe the driver allocates dummy I2C device for companion chip
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 regmap_init_i2c().

If i2c_new_dummy() fails for companion device, 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>
[bwh: Backported to 3.2:
 - Adjust filename, context
 - Add kfree() before return, as driver is not using managed allocations]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>