wireless: mwifiex: Use the proper interfaces

Why is converting time formats so desired if there are proper
interfaces for this?

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Bing Zhao <bzhao@marvell.com>
Cc: "John W. Linville" <linville@tuxdriver.com>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>