tty: frv, remove unused serial macros

STD_COM_FLAGS needs not be defined as it is not used anywhere on frv.

SERIAL_PORT_DFNS is defined to be empty. 8250 is aware of empty
SERIAL_PORT_DFNS and does:
 #ifndef SERIAL_PORT_DFNS
 #define SERIAL_PORT_DFNS
 #endif

So no need to define it on frv.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>