staging: wlan-ng: rename DIDmib_p2_p2MAC_p2CurrentTxRate in p80211metadef.h

Rename DIDmib_p2_p2MAC_p2CurrentTxRate in p80211metadef.h to
DIDMIB_P2_MAC_CURRENTTXRATE to fix "Avoid CamelCase" message from
checkpatch and conform to the coding style guidelines. Also shorten
name by removing repeated use of "P2" to ease readability and reduce
long lines.

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