serial: 8250_dw: Rework dw8250_handle_irq() locking and IIR handling

serial: 8250_dw: Rework dw8250_handle_irq() locking and IIR handling

dw8250_handle_irq() takes port's lock multiple times with no good
reason to release it in between and calls serial8250_handle_irq()
that also takes port's lock.

Take port's lock only once in dw8250_handle_irq() and use
serial8250_handle_irq_locked() to avoid releasing port's lock in
between.

As IIR_NO_INT check in serial8250_handle_irq() was outside of port's
lock, it has to be done already in dw8250_handle_irq().

DW UART can, in addition to IIR_NO_INT, report BUSY_DETECT (0x7) which
collided with the IIR_NO_INT (0x1) check in serial8250_handle_irq()
(because & is used instead of ==) meaning that no other work is done by
serial8250_handle_irq() during an BUSY_DETECT interrupt.

This allows reorganizing code in dw8250_handle_irq() to do both
IIR_NO_INT and BUSY_DETECT handling right at the start simplifying
the logic.

Tested-by: Bandal, Shankar <shankar.bandal@intel.com>
Tested-by: Murthy, Shanth <shanth.murthy@intel.com>
Cc: stable <stable@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://patch.msgid.link/20260203171049.4353-5-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>