drivers: staging: dgrp: using time_is_before_jiffies() instead of open coding 'jiffies'

Need use the proper jiffy comparison macros/functions instead of open
coding 'jiffies'.

The original 'comparation' is always true.

The related warning (with 'EXTRA_CFLAGS=-W')
  drivers/staging/dgrp/dgrp_net_ops.c:1610:10: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>