iwlwifi: mvm: fix NVM check for 3168 devices

We had a check on !NVM_EXT and then a check for NVM_SDP in the else
block of this if.  The else block, obviously, could only be reached if
using NVM_EXT, so it would never be NVM_SDP.

Fix that by checking whether the nvm_type is IWL_NVM instead of
checking for !IWL_NVM_EXT to solve this issue.

Reported-by: Stefan Sperling <stsp@stsp.name>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>