LinuxKPI: 802.11: initialize the passed in chandef in cfg80211_chandef_create

cfg80211_chandef_create() gets passed a pointer to a cfg80211_chan_def.
It seems that several users are passing in an uninitialized variable
from the stack and expect cfg80211_chandef_create() to initialize it.
Run memset() on the struct, which for all callers currently seems to do
the right thing, to avoid later accesses to uninitialized struct members
like "punctured".

Reported by: CI (gcc build)
MFC after: 3 days
Sponsonred by: The FreeBSD Foundation