net: moxa: clear DESC1 on ndo_start_xmit()

TX buffer length is not cleared on ndo_start_xmit().
Failing to do so can bug/hang the controller and
cause TX interrupts to stop altogether.

Remove the readl() and compute a new value for DESC1.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=69031

Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>