uart: ns16550: add workaround to re-enable interrupts in ISR

In some configurations (e.g. edge interrupt triggers),
an interruptible event occurs during ISR and the host interrupt
controller does not see the new event due to IIR is constantly
asserting interrupts. For example, the callback handles RX and
then TX. If another character comes in before end of TX processing
(TX interrupt still asserts while raising RX interrupt), the host
interrupt controller may not see this new event. So if needed,
the IER is being toggled to re-assert interrupts at the end of ISR
to nudge the host interrupt controller to fire the ISR again.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>