Revert "PCI: Avoid race while enabling upstream bridges"

This reverts commit 40f11adc.

Jens found that iwlwifi firmware loading failed on a Lenovo X1 Carbon,
gen4:

  iwlwifi 0000:04:00.0: Direct firmware load for iwlwifi-8000C-34.ucode failed with error -2
  iwlwifi 0000:04:00.0: Direct firmware load for iwlwifi-8000C-33.ucode failed with error -2
  iwlwifi 0000:04:00.0: Direct firmware load for iwlwifi-8000C-32.ucode failed with error -2
  iwlwifi 0000:04:00.0: loaded firmware version 31.532993.0 op_mode iwlmvm
  iwlwifi 0000:04:00.0: Detected Intel(R) Dual Band Wireless AC 8260, REV=0x208
  ...
  iwlwifi 0000:04:00.0: Failed to load firmware chunk!
  iwlwifi 0000:04:00.0: Could not load the [0] uCode section
  iwlwifi 0000:04:00.0: Failed to start INIT ucode: -110
  iwlwifi 0000:04:00.0: Failed to run INIT ucode: -110

He bisected it to 40f11adc ("PCI: Avoid race while enabling upstream
bridges").  Revert that commit to fix the regression.

Link: http://lkml.kernel.org/r/4bcbcbc1-7c79-09f0-5071-bc2f53bf6574@kernel.dk
Fixes: 40f11adc

 ("PCI: Avoid race while enabling upstream bridges")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Srinath Mannam <srinath.mannam@broadcom.com>
CC: Jens Axboe <axboe@kernel.dk>
CC: Luca Coelho <luca@coelho.fi>
CC: Johannes Berg <johannes@sipsolutions.net>
CC: Emmanuel Grumbach <emmanuel.grumbach@intel.com>