Downgrade tty_intr_event from a global

It can be static within uart_tty.c. It is an open question whether there
remains any real benefit to having uart instances share a swi thread.

Reviewed by: imp, markj, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36938

(cherry picked from commit 05b727fee550598bfae5754783f9decfdeeb3c3d)