staging: wlan-ng: convert P80211SKB_RXMETA to inline function in p80211conv

To avoid possible issues with repeated reference to the macro argument
as reported by checkpatch, macro P80211SKB_RXMETA is replaced with an
equivalent inline function. The function is named p80211skb_rxmeta to
follow the coding style guidelines; references to the macro are
updated to reference the new function.

This change depends on the similar change for P80211SKB_FRMMETA having
been applied.

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