iwlwifi: pnvm: move file loading code to a separate function

In preparation to support loading the PNVM from UEFI, move the
function that loads the PNVM from the filesystem to a separate
function.  This will make it easier to try to load from both places
later on.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210210172142.072aa2e8bbc5.Ib351ee5da47a4cee60d44e66d32d2f6bba6f3150@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>