ns16550: fix ISR lockup on Allwinner uart

ns16550: fix ISR lockup on Allwinner uart

This patch fixes an ISR lockup seen on Allwinner uart

On Allwinner H5, serial driver goes into an infinite loop
when interrupts are enabled. The reason is a residual
"busy detect" interrupt. Since the condition UART_IIR_NOINT
will not be true unless this interrupt is cleared, the
interrupt handler will remain locked up in this while loop.

A HW quirk fix was previously added for designware uart under
commit:
50417cd978aa54930d065ac1f139f935d14af76d

It checks for a busy condition during setup and clears the
condition by reading UART_USR register.

On Allwinner hardware, the "busy detect" condition occurs
later because an LCR write is performed during setup 'after'
this clear and if uart is busy, the "busy detect" condition
will trigger again and cause the ISR lockup.

To solve this problem, the same UART_USR read operation needs
to be performed within the interrupt handler to clear this
condition.

Linux dw 8250 driver also handles this condition within
interrupt handler
http://elixir.free-electrons.com/linux/latest/source/drivers/tty/serial/8250/8250_dw.c#L233

Tested on Orange Pi PC2 (H5). This issue is seen on H3
as well and the same fix works.

Signed-off-by: Awais Masood <awais.masood@vadion.com>
Acked-by: Jan Beulich <jbeulich@suse.com>