platform/x86: surface3_power: Fix a NULL vs IS_ERR() check in probe

The i2c_acpi_new_device() function never returns NULL, it returns error
pointers.

Fixes: b1f81b49 ("platform/x86: surface3_power: MSHW0011 rev-eng implementation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>