cfg80211: fix reporting 5/10 MHz support to user space

nla_put_flag needs a real nl80211 attribute id, not a wiphy flag bit.
While at it, split 5 and 10 MHz capability flags in case we ever need
to support hardware that can only do one of the two.

Also move the flag settings to the split-only information so we don't
increase the space needed for old userspace.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
[change location of flag setting]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>