iwlwifi: pcie: fix ucode load flow for a000 devices

Turns out we should access TFH relative addresses.
Also, the FH_UCODE_LOAD_STATUS was replaced by
UREG_UCODE_LOAD_STATUS.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>