staging: wlan-ng: convert P80211SKB_FRMMETA to inline function in p80211conv

To avoid possible issues with repeated reference to the macro argument
as reported by checkpatch, macro P80211SKB_FRMMETA is replaced with an
equivalent inline function. The function is named p80211skb_frmmeta to
follow the coding style guidelines; references to the macro are
updated to reference the new function.

Signed-off-by: Tim Collier <osdevtc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>