wil6210: fix smatch warning in wil_cfg80211_get_station()

Smatch suggests to propagate error code from wil_find_cid(), and, indeed,
it is a good idea.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>