Workaround dropped notifications in the iir register.  Register reads
coincident with new interrupt notifications sometimes result in this
device clearing the interrupt event without reporting it in the read
data.

The serial core already has a heuristic for determining when a device
has an untrustworthy iir register.  In this case when we apriori know
that the iir is faulty use a flag (UPF_BUG_THRE) to bypass the test and
force usage of the background timer.

[stable: 3.3.x]
Acked-by: NAlan Cox <alan@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Reported-by: NNhan H Mai <nhan.h.mai@intel.com>
Reported-by: NSudhakar Mamillapalli <sudhakar@fb.com>
Tested-by: NNhan H Mai <nhan.h.mai@intel.com>
Tested-by: NSudhakar Mamillapalli <sudhakar@fb.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>