drm/xe: Fix a NULL vs IS_ERR() bug in xe_i2c_register_adapter()

The fwnode_create_software_node() function returns error pointers.  It
never returns NULL.  Update the checks to match.

Fixes: f0e53aad ("drm/xe: Support for I2C attached MCUs")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/65825d00-81ab-4665-af51-4fff6786a250@sabinyo.mountain


Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>