ipw2x00: Remove redundant variable "rc"

local variable "rc" is not used. It is safe to remove and
There is only one caller of libipw_qos_convert_ac_to_parameters().
hence make it void

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>