usb: typec: ucsi: acpi: fix a NULL vs IS_ERR() check in probe

The devm_memremap() function never returns NULL.  It returns error
pointers.

Fixes: cdc3d2ab ("usb: typec: ucsi: acpi: Map the mailbox with memremap()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>