Instead of open coding the capability structure search, find the PM
Capability using pci_find_capability().

While at it, rename the generic 'cap_pointer' to 'pm_cap' which makes
the intent of the code more obvious.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231124084725.12738-7-ilpo.jarvinen@linux.intel.com