brcmsmac: Use DIV_ROUND_CLOSEST directly to make it readable

The kernel.h macro DIV_ROUND_CLOSEST performs the computation (x + d/2)/d
but is perhaps more readable.

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