iwlwifi: refactor the code that reads the MAC address from the NVM

It makes it slightly easier to follow. Pass the pointer to
the transport which allows to read WFMP_MAC_ADDR_X register
only when needed and to use IWL_ERR instead of the less
commonly used IWL_ERR_DEV logger macro.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>