net: wireless: fix misplaced #endif in net/wireless/nl80211.c

The patch "nl80211: cumulative vendor command support patch" introduced
compilation error in file net/wireless/nl80211.c. The nl80211_vendor_mcgrp
variable is defined only if the CONFIG_NL80211_TESTMODE preprocessor constant
is defined. However, this variable is later used wether
CONFIG_NL80211_TESTMODE is defined or not. The cause is a misplaced #endif.

Change-Id: I466488285578d57e6554a1f8ebe71d4f3385ecf2
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>