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

This reverts commit a91a3c2d 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