serial: 8250_mid: handle interrupt correctly in DMA case

Starting from Tangier B0 and continuing on Anniedale the HSU DMA interrupt
line is actually shared with UART. Handling them independently is racy and
quite often comes with the following traceback.

 irq 54: nobody cared (try booting with the "irqpoll" option)
 CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.9.0-rc6-edison64-86244934+ #1
 Hardware name: Intel Corporation Merrifield/BODEGA BAY, BIOS 542 2015.01.21:18.19.48
  ffff88003f203eb0 ffffffff8130e718 ffff880032627000 ffff88003262709c
  ffff88003f203ed8 ffffffff810a3960 ffff880032627000 0000000000000000
  ffff880032627000 ffff88003f203f10 ffffffff810a3cc7 ffff880032627000
 Call Trace:
  <IRQ>
  [<ffffffff8130e718>] dump_stack+0x4d/0x65
  [<ffffffff810a3960>] __report_bad_irq+0x30/0xc0
  [<ffffffff810a3cc7>] note_interrupt+0x227/0x270
  [<ffffffff810a1380>] handle_irq_event_percpu+0x40/0x50
  [<ffffffff810a13b7>] handle_irq_event+0x27/0x50
  [<ffffffff810a42d5>] handle_fasteoi_irq+0x85/0x150
  [<ffffffff8101d7fe>] handle_irq+0x6e/0x120
  [<ffffffff8105b8bc>] ? _local_bh_enable+0x1c/0x50
  [<ffffffff8101d0d6>] do_IRQ+0x46/0xd0
  [<ffffffff818cef3f>] common_interrupt+0x7f/0x7f
  <EOI>
  [<ffffffff818cdead>] ? mwait_idle+0x7d/0x140
  [<ffffffff81024c9a>] arch_cpu_idle+0xa/0x10
  [<ffffffff818ce150>] default_idle_call+0x20/0x30
  [<ffffffff810908fd>] cpu_startup_entry+0x16d/0x1d0
  [<ffffffff818c882d>] rest_init+0x6d/0x70
  [<ffffffff81f93e8f>] start_kernel+0x3e2/0x3ef
  [<ffffffff81f9343d>] x86_64_start_reservations+0x38/0x3a
  [<ffffffff81f93529>] x86_64_start_kernel+0xea/0xed
 handlers:
 [<ffffffff81411670>] serial8250_interrupt
 Disabling IRQ #54

Fix this by handling interrupt only in one place.

The issue is discussed here: https://github.com/andy-shev/linux/issues/5



Moreover this also fixes another bug when Rx DMA returns wrong residue and we
can't rely on it.

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>