If an existing "wpa_psk_file" is passed to hostapd, the "key" option may
be omitted.
While we're at it, also improve the passphrase length checking to ensure
that it is either exactly 64 bytes or 8 to 63 bytes.
Fixes: FS#2689
Ref: https://github.com/openwrt/openwrt/pull/3283
Suggested-by: Michael Jones <mike@meshplusplus.com>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>