hostapd: fix stray "out of range" shell errors in hostapd.sh

The hostapd_append_wpa_key_mgmt() procedure uses the possibly uninitialized
$ieee80211r and $ieee80211w variables in a numerical comparisation, leading
to stray "netifd: radio0 (0000): sh: out of range" errors in logread when
WPA-PSK security is enabled.

Ensure that those variables are substituted with a default value in order to
avoid emitting this (harmless) shell error.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>