brcmfmac: fix memory leak in brcmf_cfg80211_attach()

In brcmf_cfg80211_attach() there was one error path not properly
handled as it leaked memory allocated in brcmf_btcoex_attach().

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>