r6040: fix null pointer access and tx timeouts

This patch fixes a null pointer access in r6040_rx due
to lp->dev not being initialized.

Fix the TX timeouts, TX irq was not re-enabled on RX irq

Signed-Off-By: Joerg Albert <jal2@gmx.de>
Signed-off-by: Florian Fainelli <florian.fainelli@telecomint.eu>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>