cfg80211: remove unnecessary include clauses (cfg80211.h)

<linux/if_ether.h> is included by <linux/ieee80211.h>;
<linux/skbuff.h> is included by <linux/if_ether.h>;
<linux/bug.h> and <linux/net.h> is included by <linux/skbuff.h>.

So just remove them.
mac80211: remove unnecessary include clauses (mac80211.h)

<linux/bug.h>, <linux/kernel.h>, <linux/if_ether.h>, <linux/skbuff.h>
and <linux/ieee80211.h> are all directly or indirectly included by
<net/cfg80211.h>.

So just remove them.
mac80211: remove unnecessary include clauses
cfg80211: remove unnecessary include clauses
mac80211: slightly refactor ieee80211_reconfig()

Only try to update chanctx if local->use_chanctx is true.

P2P_DEVICE doesn't support ieee80211_bss_info_change_notify() for now,
so it's not needed to set changed flags for P2P_DEVICE. Besides, flag
BSS_CHANGED_IDLE is already in the common change flags.
mac80211: remove config check

Sanity check and error handling code shouldn't be put into
CONFIG_MAC80211_VERBOSE_DEBUG block, since verbose debug option is
used to print debug messages.
mac80211: return bool instead of numbers for yes or no function

And some code style change, and correct a typo in comment.
mac80211: remove BUILD_BUG_ON check

Both ht_cap and sband->ht_cap is a instance of struct
ieee80211_sta_ht_cap ht_cap, and the struct has fixed size, so the
check may be unnecessary ?
cfg80211: change pointer name

Name dev is too common and ambiguous, change all the pointer name
which points to struct cfg80211_registered_device to rdev. This can
increase the code readability.