mt76: mt7915: add missing CONFIG_MAC80211_DEBUGFS

Add CONFIG_MAC80211_DEBUGFS to fix a reported warning.

Fixes: ec9742a8

 ("mt76: mt7915: add .sta_add_debugfs support")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>