libexec/rc.d/hostapd: Down/up interface when interface is specified

When no interface is specified results in a syntax error in the rc
script. Only execute poststart when an interface has been specified.

PR: 263358
Submitted by: markj
Reported by: Joshua Kinard <freebsd@kumba.dev>
Fixes: 0da2c91e64528d896f69d36670e25b4b4a140579

(cherry picked from commit 1452bfcd9bbcb2f5bbb89fa38d01ce51dd9b6d44)