netifd: fix start_disabled 'sh: bad number' in mac80211 setup (bug #19345)

Two errors "netifd: radio0: sh: bad number" have recently surfaced in system
log in trunk when wifi interfaces come up. I tracked the errors to checking
numerical values of some config options without ensuring that the option has
any value.

The errors I see have apparently been introduced by r45051 (ieee80211r in
hostapd) and r45326 (start_disabled in mac80211). My patches fix two
instances of "bad number", but there may be a third one, as the original
report in bug 19345 pre-dates r45326 and already has two "bad number" errors
for radio0.

https://dev.openwrt.org/ticket/19345



Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

SVN-Revision: 45379