mwifiex: use time_after()

To be future-proof and for better readability the time comparisons are
modified to use time_after() instead of plain, error-prone math.

Signed-off-by: Manuel Schölling <manuel.schoelling@gmx.de>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>