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>

drivers/staging/dgrp/dgrp_net_ops.c

@@ -1607,7 +1607,7 @@ static int dgrp_send(struct nd_struct *nd, long tmax)
 					if ((ch->ch_pun.un_flag & UN_LOW) != 0 ?
 					    (n <= TBUF_LOW) :
 					    (ch->ch_pun.un_flag & UN_TIME) != 0 ?
-					    ((jiffies - ch->ch_waketime) >= 0) :
+					    time_is_before_jiffies(ch->ch_waketime) :
 					    (n == 0 && ch->ch_s_tpos == ch->ch_s_tin) &&
 					    ((ch->ch_pun.un_flag & UN_EMPTY) != 0 ||
 					    ((ch->ch_tun.un_open_count &&