mwifiex: Fix mem leak in mwifiex_tm_cmd

'hostcmd' is alloced by kzalloc, should be freed before
leaving from the error handling cases, otherwise it will
cause mem leak.

Fixes: 3935ccc1 ("mwifiex: add cfg80211 testmode support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>