module: make __mod_device_table__* symbols static

The __mod_device_table__* symbols are only parsed by modpost to generate
MODULE_ALIAS() entries from MODULE_DEVICE_TABLE().

Therefore, these symbols do not need to be globally visible, or globally
unique.

If they are in the global scope, we would worry about the symbol
uniqueness, but modpost is fine with parsing multiple symbols with the
same name.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>