tty/metag_da: Avoid module_init/module_exit in non-modular code

The metag_da TTY driver can't get built as a module at the moment, but
it still uses module_init() and module_exit(). Those macros are moving
to module.h which isn't included by metag_da.c, which will result in the
following build warnings (remarkably no build errors) and an apparent
failure to boot as the TTY driver won't be loaded.

drivers/tty/metag_da.c:660: warning: data definition has no type or storage class
drivers/tty/metag_da.c:660: warning: type defaults to ‘int’ in declaration of ‘module_init’
drivers/tty/metag_da.c:660: warning: parameter names (without types) in function declaration
drivers/tty/metag_da.c:661: warning: data definition has no type or storage class
drivers/tty/metag_da.c:661: warning: type defaults to ‘int’ in declaration of ‘module_exit’
drivers/tty/metag_da.c:661: warning: parameter names (without types) in function declaration
drivers/tty/metag_da.c:572: warning: ‘dashtty_init’ defined but n...