iwlwifi: pnvm: implement reading PNVM from UEFI

commit a1a6a4cf

 upstream.

We now support fetching the PNVM data from a UEFI variable.  Add the
code to read this variable first and use it.  If it's not available,
we fall back to reading the data from the filesystem, as before.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210211015026.289084803334.Ie234805047df3be84f4235f9dafaf4cdecf0db9a@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>