When starting an AP or IBSS network, reconfigure the operation mode to

When starting an AP or IBSS network, reconfigure the operation mode to
11ac/n/a/g/b as applicable. Fixes an issue where hostap would end up
running in the rather meaningless MODE_AUTO unless a mode was explicitly
configured with ifconfig.

Found while investigating issues reported by trondd testing my athn RA patch.

ok deraadt@