drivers/net/r6040.c: correct bad use of round_jiffies()

Compared to other places in the kernel, I think that this driver misuses
the function round_jiffies.

Signed-off-by: Christophe Jaillet <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>