platform/loongarch: laptop: Unregister generic_sub_drivers on exit

Without correct unregisteration, ACPI notify handlers and the platform
drivers installed by generic_subdriver_init() will become dangling
references after removing the loongson_laptop module, triggering various
kernel faults when a hotkey is sent or at kernel shutdown.

Cc: stable@vger.kernel.org
Fixes: 6246ed09 ("LoongArch: Add ACPI-based generic laptop driver")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>