wifi: rsi: Fix memory leak in rsi_coex_attach()

The coex_cb needs to be freed when rsi_create_kthread() failed in
rsi_coex_attach().

Fixes: 2108df3c ("rsi: add coex support")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221205061441.114632-1-yuancan@huawei.com