Revert "serial: 8250: Toggle IER bits on only after irq has been set up"

This reverts commit 572fb97f which is
commit 039d4926 upstream.

It breaks the kernel abi and is not needed for any supported Android
devices at this point in time, so it can be safely reverted.

Bug: 161946584
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I07e784e722762dab69f031063317350379e59b76