ieee80211: Increase PMK maximum length to 64 bytes

Increase the PMK maximum length to 64 bytes to accommodate
the key length used in DPP with the NIST P-521 and
Brainpool 512 curves.

Signed-off-by: Srinivas Dasari <dasaris@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>