net: lantiq_xrx200: increase buffer reservation

If the user sets a lower mtu on the CPU port than on the switch,
then DMA inserts a few more bytes into the buffer than expected.
In the worst case, it may exceed the size of the buffer. The
experiments showed that the buffer should be a multiple of the
burst length value. This patch rounds the length of the rx buffer
upwards and fixes this bug. The reservation of FCS space in the
buffer has been removed as PMAC strips the FCS.

Fixes: 998ac358 ("net: lantiq: add support for jumbo frames")
Reported-by: Thomas Nixon <tom@tomn.co.uk>
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>