platform/x86: serdev_helpers: Check for serial_ctrl_uid == NULL

platform/x86: serdev_helpers: Check for serial_ctrl_uid == NULL

dell_uart_bl_pdev_probe() calls get_serdev_controller() with the
serial_ctrl_uid parameter set to NULL.

In case of errors this NULL parameter then gets passed to pr_err()
as argument matching a "%s" conversion specification. This leads to
compiler warnings when building with "make W=1".

Check serial_ctrl_uid before passing it to pr_err() to avoid these.

Fixes: dc5afd720f84 ("platform/x86: Add new get_serdev_controller() helper")
Cc: stable@vger.kernel.org
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20241204204227.95757-4-hdegoede@redhat.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>