iwlwifi: move UEFI code to a separate file

We are going to read more variables from UEFI, so it's cleaner to have
all the code that handles UEFI variables in a separate file.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210621103449.c705ac86f2e9.Ia7421c17fe52929e4098b4f0cf070809ed3ef906@changeid


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