staging: dgnc: Use time_after_eq()

To be future-proof and for better readability the time comparisons are
modified to use time_after_eq() instead of plain, error-prone math.

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