fsi: fsi-occ: fix return value check in occ_probe()

In case of error, the function platform_device_register_full()
returns ERR_PTR() and never returns NULL. The NULL test in the
return value check should be replaced with IS_ERR().

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Joel Stanley <joel@jms.id.au>