nl80211: fix monitor flags

NLA_NESTED attributes cannot be empty, but we want to be able to
specify "no flags" (empty attribute) vs. "no change" (no attribute).
Therefore, remove the NLA_NESTED policy so it can work as an empty
attribute.

I guess I should have used a u32 for these flags instead, but we're
stuck with it now. Haven't noticed earlier because of a bug in iw...

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>