PCI/pwrctrl: Fix the kerneldoc tag for private fields

PCI/pwrctrl: Fix the kerneldoc tag for private fields

The correct tag for marking private fields in kerneldoc is "private:", not
capitalized "Private:". Fix the pwrctl struct to silence the following
warnings:

  Warning: include/linux/pci-pwrctrl.h:45 struct member 'nb' not described in 'pci_pwrctrl'
  Warning: include/linux/pci-pwrctrl.h:45 struct member 'link' not described in 'pci_pwrctrl'
  Warning: include/linux/pci-pwrctrl.h:45 struct member 'work' not described in 'pci_pwrctrl'

Fixes: 4565d2652a37 ("PCI/pwrctl: Add PCI power control core code")
Reported-by: Bjorn Helgaas <helgaas@kernel.org>
Closes: https://lore.kernel.org/all/20250617233539.GA1177120@bhelgaas/
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250618091129.44810-1-brgl@bgdev.pl