bus: mhi: core: Fix a NULL vs IS_ERR check in mhi_create_devices()

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

Fixes: da1c4f85 ("bus: mhi: core: Add support for creating and destroying MHI devices")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20200407093133.GM68494@mwanda


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>