Downgrade tty_intr_event from a global

[mhorne]

Summary

It can be static within uart_tty.c.

[jhb]

sys/dev/uart/uart_tty.c

80

Is this even a useful thing to do now vs just having per-instance threads? In pre-SMPng we had limited bits for SWIs.

[imp]

sys/dev/uart/uart_tty.c

80

This change is fine, but would be redundant if we did that :)

[mhorne]

sys/dev/uart/uart_tty.c

80

I'll make a note of this notion in the commit message, but in my own self-interest I will avoid making that behaviour change :)