mwifiex: remove redundant timestamp assignment

During AMSDU aggregation, we are already using timestamp
value of a first packet being aggregated. This patch removes
redundant ktime_get_real() call.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>