void __init sn_sal_switch_to_interrupts (struct sn_cons_port * port);
In sn_sal_module_init, after we're registered with serial core and the port is added, this function is called to switch us to interrupt mode. We were previously in asynch/polling mode (using init_timer).
We attempt to switch to interrupt mode here by calling sn_sal_connect_interrupt. If that works out, we enable receive interrupts.