[WAN]: drivers/net/wan/: use of time_after macro

From: Marcelo Feitoza Parisi <marcelo@feitoza.com.br>

Use of the time_after() macro, defined at linux/jiffies.h, which deal
with wrapping correctly and are nicer to read.

Signed-off-by: Marcelo Feitoza Parisi <marcelo@feitoza.com.br>
Signed-off-by: Domen Puncer <domen@coderock.org>
Signed-off-by: David S. Miller <davem@davemloft.net>