staging: wilc1000: use WLAN_PMKID_LEN macro from ieee80211.h header

Make use of WLAN_PMKID_LEN macro provided by ieee80211.h header instead
of PMKID_LEN.

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>