Enable 802.11g hardware mode in hostapd.conf

The current hostapd.conf generated by SoftapController does not
contain a hw_mode setting.  As a result hostapd defaults to 802.11b
mode.  This mode is inconsistent with the setting "ieee80211n=1" and
prevents hostapd from using 802.11g/n rates.  Add "hw_mode=g" to the
generated hostapd.conf to allow 802.11g/n rates to be advertised and
used.

Change-Id: Ifd66da3f3ffdfffb7c28e751d20bc432e070b2c3