iwlwifi: pcie: fix erroneous "Read failed message"

Current pci dumping code code is always falling to the error
path, resulting with a constant "Read failed" message, also
for the successful reads.

Fixes: a5c932e41fdd ("iwlwifi: pcie: dump registers when HW becomes inaccessible")
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>