ntb: intel: Fix error handling in intel_ntb_pci_driver_init()

A problem about ntb_hw_intel create debugfs failed is triggered with the
following log given:

 [  273.112733] Intel(R) PCI-E Non-Transparent Bridge Driver 2.0
 [  273.115342] debugfs: Directory 'ntb_hw_intel' with parent '/' already present!

The reason is that intel_ntb_pci_driver_init() returns
pci_register_driver() directly without checking its return value, if
pci_register_driver() failed, it returns without destroy the newly created
debugfs, resulting the debugfs of ntb_hw_intel can never be created later.

 intel_ntb_pci_driver_init()
   debugfs_create_dir() # create debugfs directory
   pci_register_driver()
     driver_register()
       bus_add_driver()
         priv = kzalloc(...) # OOM happened
   # return without destroy debugfs directory

Fix by removing debugfs when pci_register_driver() returns error.

Fixes: e26a5843

 ("NTB: Split ntb_hw_intel and ntb_transport drivers")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>