pci_disable_device() called in __ixgbe_shutdown() decreases
dev->enable_cnt by 1. pci_enable_device_mem() which increases
dev->enable_cnt by 1, was removed from ixgbe_resume() in commit
6f82b255 ("ixgbe: use generic power management"). This caused
unbalanced increase/decrease. So add pci_enable_device_mem() back.

Fix the following call trace.

  ixgbe 0000:17:00.1: disabling already-disabled device
  Call Trace:
   __ixgbe_shutdown+0x10a/0x1e0 [ixgbe]
   ixgbe_suspend+0x32/0x70 [ixgbe]
   pci_pm_suspend+0x87/0x160
   ? pci_pm_freeze+0xd0/0xd0
   dpm_run_callback+0x42/0x170
   __device_suspend+0x114/0x460
   async_suspend+0x1f/0xa0
   async_run_entry_fn+0x3c/0xf0
   process_one_work+0x1dd/0x410
   worker_thread+0x34/0x3f0
   ? cancel_delayed_work+0x90/0x90
   kthread+0x14c/0x170
   ? kthread_park+0x90/0x90
   ret_from_fork+0x1f/0x30

Fixes: 6f82b255 ("ixgbe: use generic power management")
Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>