ar71xx: ag71xx: calculate max frame len register value from the MTU

Set the MAX_FRAME_LEN register to zero in ag71xx_hw_init()
and write the correct value into that from the ag71xx_open()
and ag71xx_fast_reset() functions.

Also recalculate the RX buffer size based on the actual
maximum frame length value to optimize memory allocation.
Additionaly, disallow to change the MTU value while the
interface it running.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39147 3c298f89-4303-0410-b956-a3cf2f4a3e73