brcmfmac: fix memory leak in brcmf_fill_bss_param

This function is called from get_station callback which means that every
time user space was getting/dumping station(s) we were leaking 2 KiB.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Fixes: 1f0dc59a ("brcmfmac: rework .get_station() callback")
Cc: stable@vger.kernel.org # 4.2+
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>