pmdomain: thead: Fix error pointer vs NULL bug in th1520_pd_reboot_init()

The devm_auxiliary_device_create() returns NULL on error.  It never
returns error pointers.  Using PTR_ERR_OR_ZERO() here means the function
always returns success.  Replace the PTR_ERR_OR_ZERO() call check with
a NULL check.

Fixes: 64581f41 ("pmdomain: thead: create auxiliary device for rebooting")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Icenowy Zheng <uwu@icenowy.me>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>