staging: rtl8188eu: Remove "last_tx_time" from struct sreset_priv

value stored in variable last_tx_time isn't being used
by driver.

Signed-off-by: navin patidar <navin.patidar@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>