iwlwifi: mvm: Do not require PHY_SKU NVM section for 3168 devices

The logic for checking required NVM sections was recently fixed in
commit b3f20e09 ("iwlwifi: mvm: fix NVM check for 3168
devices"). However, with that fixed the else is now taken for 3168
devices and within the else clause there is a mandatory check for the
PHY_SKU section. This causes the parsing to fail for 3168 devices.

The PHY_SKU section is really only mandatory for the IWL_NVM_EXT
layout (the phy_sku parameter of iwl_parse_nvm_data is only used when
the NVM type is IWL_NVM_EXT). So this changes the PHY_SKU section
check so that it's only mandatory for IWL_NVM_EXT.

Fixes: b3f20e09

 ("iwlwifi: mvm: fix NVM check for 3168 devices")
Signed-off-by: Dan Moulding <dmoulding@me.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>