brcmfmac: reject too long PSK

nl80211 already allows specifying 48 bytes, but brcmfmac
only supports 32. Reject keys that are too long.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>