mips: Loongson: Fix the link time qualifier of 'serial_exit()'

'exit' functions should be marked as __exit, not __init.

Fixes: 85cc0288 ("mips: make loongsoon serial driver explicitly modular")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: chenhc@lemote.com
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-janitors@vger.kernel.org