net: ravb: Fix RX frame size limit

net: ravb: Fix RX frame size limit

The value written to the RFLR register includes the length of the CRC
data at the end of each Ethernet frame. So we need to increase the value
written to this register to ensure that we can receive full size frames.

While we're here we can also copy the improved comment from the Linux
kernel.

Fixes: 8ae51b6f324e ("net: ravb: Add Renesas Ethernet RAVB driver")
Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org> # Fix comment
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>