iwlwifi: move get pnvm file name to a separate function

Move code that generates the pnvm file name to a separate function,
so that it can be reused.

Signed-off-by: Dror Moshe <drorx.moshe@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210826224715.7d2dd18c75a2.I3652584755b9ab44909ddcd09ff4d80c6690a1ad@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>