iwlwifi: don't leak memory on allocation failure

If we fail to allocate the small chunk of memory for the
pieces of the firmware file, we leak the whole firmware
image instead...

Since the allocation failure is really unlikely, just bail
out at that point instead.

Remove the error message at the label since we now (and
actually have been) use it for various reasons.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>