wifi: iwlwifi: fix double-free bug

In the Linux kernel, the following vulnerability has been resolved:

wifi: iwlwifi: fix double-free bug

The storage for the TLV PC register data wasn't done like all the other storage in the drv->fw area, which is cleared at the end of deallocation. Therefore, the freeing must also be done differently, explicitly NULL'ing it out after the free, since otherwise there's a nasty double-free bug here if a file fails to load after this has been parsed, and we get another free later (e.g. because no other file exists.) Fix that by adding the missing NULL assignment.