The process of probing the 8250 serial port is as follows:

1. Start detecting IRQs

2. Enable the IER register [At this point, the port is supposed to light
   the INTR].

3. Stop detecting IRQs [At this point, the driver detects which IRQ
   belongs to that port].

4. Disable IER register.

Since we weren't enabling and disabling the IRQ based on IER writes, we
would often fail the probing since the driver couldn't detect which IRQ
is used by the port, and would just default that to 0.

This would cause slowness and may have caused hangs. For me there is a
significant increase in speed of the terminal after this patch.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
[ penberg@kernel.org: whitspace cleanup ]
Signed-off-by: Pekka Enberg <penberg@kernel.org>