mips: make loongsoon serial driver explicitly modular

The file looks as if it is non-modular, but it piggy-backs
off CONFIG_SERIAL_8250 which is tristate.  If set to "=m"
we will get this after the init/module header cleanup:

arch/mips/loongson/common/serial.c:76:1: error: data definition has no type or storage class [-Werror]
arch/mips/loongson/common/serial.c:76:1: error: type defaults to 'int' in declaration of 'device_initcall' [-Werror=implicit-int]
arch/mips/loongson/common/serial.c:76:1: error: parameter names (without types) in function declaration [-Werror]
arch/mips/loongson/common/serial.c:58:19: error: 'serial_init' defined but not used [-Werror=unused-function]
cc1: all warnings being treated as errors
make[3]: *** [arch/mips/loongson/common/serial.o] Error 1

Make it clearly modular, and add a module_exit function,
so that we avoid the above breakage.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>