staging: rtl8188eu: use time_before/time_after and change type of ips_deny_time

To be future-proof and for better readability the time comparisons are
modified to use time_before/_after() instead of plain, error-prone math.
To suppress compiler warnings the type of ips_deny_time was changed.

Signed-off-by: Manuel Schölling <manuel.schoelling@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>