ntb: ntb_hw_intel: link_poll isn't clearing the pending status properly

On Skylake hardware, the link_poll isn't clearing the pending interrupt
bit.  Adding a new function for SKX that handles clearing of status bit the
right way.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Fixes: 783dfa6c ("ntb: Adding Skylake Xeon NTB support")
Signed-off-by: Jon Mason <jdmason@kudzu.us>