iwlwifi: mvm: ring the doorbell and wait for PNVM load completion

When we receive a non-zero SKU_ID in the alive notification, we need
to ring the doorbell and wait for the FW to send us a PNVM load
complete notification before we continue the init phase.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20201008180656.a10e8b6bbcf9.Ib5d10b3d508a4d2d4e6b7b629af89d76f4f03d81@changeid