iwlwifi: pcie: prepare the device before accessing it

For 8000 series, we need to access the device to know what
firmware to load. Before we do so, we need to prepare the
device otherwise we might not be able to access the
hardware.

Fixes: c278754a21e6 ("iwlwifi: mvm: support family 8000 B2/C steps")
CC: <stable@vger.kernel.org> [4.1]
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>