mac80211: don't call md5sum on non-existing file

If no AP is configured, hostapd-${phy}.conf is not being created,
hence md5sum fails and causes log pollution:

netifd: radio1 (3183): md5sum: can't open '/var/run/hostapd-phy1.conf': No such file or directoy

Hence make sure the file exists when calling md5sum.

Fixes: a5bc9787 ("mac80211: add support for dynamically reconfiguring wifi")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>