Revert "mwifiex: Use the proper interfaces"

This reverts commit a82fc3b4a2bceb7c6587249cb690342eb5065979.

Thomas corrected me on that I misunderstood Johannes' comment
for net_timedelta() and the ktime_get_real() usage inside
__net_timestamp().

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>