wil6210: check rx_buff_mgmt before accessing it

commit d6a553c0c61b0b0219764e4d4fc14e385085f374 upstream.

Make sure rx_buff_mgmt is initialized before accessing it.

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>