MFC: r285843 (r286059 in stable/10)

MFC: r285843 (r286059 in stable/10)

- Since r253161, uart_intr() abuses FILTER_SCHEDULE_THREAD for signaling
  uart_bus_attach() during its test that 20 iterations weren't sufficient
  for clearing all pending interrupts, assuming this means that hardware
  is broken and doesn't deassert interrupts. However, under pressure, 20
  iterations also can be insufficient for clearing all pending interrupts,
  leading to a panic as intr_event_handle() tries to schedule an interrupt
  handler not registered. Solve this by introducing a flag that is set in
  test mode and otherwise restores pre-r253161 behavior of uart_intr(). The
  approach of additionally registering uart_intr() as handler as suggested
  in PR 194979 is not taken as that in turn would abuse special pccard and
  pccbb handling code of intr_event_handle(). [1]
- Const'ify uart_driver_name.
- Fix some minor style bugs.

PR:		194979 [1]
Reviewed by:	marcel (earlier version)
Approved by:	re (gjb)

git-svn-id: https://svn.freebsd.org/base/releng/10.2@286061 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f