mac80211: add default value for noscan

mac80211: add default value for noscan

Commit b3d8b3a introduced a new test:

[ -n "$noscan" -a "$noscan" -gt 0 ] && hostapd_noscan=1

But if length of "$noscan" is zero (noscan is not set) this doesn't stop
the shell to evaluate the rest of the test.

root@hank2:~# [ -n "$noscan" -a "$noscan" -gt 0 ]
ash: out of range
root@hank2:~#

So when radios are brought up this shows in the log:

Sat Nov 23 10:51:38 2019 daemon.info procd: - init complete -
Sat Nov 23 10:52:24 2019 daemon.notice netifd: radio1 (1243): sh: out of range
Sat Nov 23 10:52:25 2019 user.notice firewall: Reloading firewall due to ifup of wan (eth0.2)
Sat Nov 23 10:52:25 2019 daemon.notice netifd: radio0 (1242): sh: out of range
Sat Nov 23 10:52:26 2019 authpriv.info dropbear[1536]: Not backgrounding

This commit sets noscan to 0 if unset and removes the gratuitous length
check, preventing the warning.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>