platform/x86: dell-uart-backlight: Make dell_uart_bl_serdev_driver static

platform/x86: dell-uart-backlight: Make dell_uart_bl_serdev_driver static

Sparse reports:

dell-uart-backlight.c:328:29: warning: symbol
'dell_uart_bl_serdev_driver' was not declared. Should it be static?

Fix it by making the symbol static.

Fixes: 484bae9e4d6ac ("platform/x86: Add new Dell UART backlight driver")
Reviewed-by: Mario Limonciello <maroi.limonciello@amd.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20250304160639.4295-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>