ar71xx: ag71xx: fix max frame length setup of the built-in switches

The currently used bitmask of the maximum frame length field
is wrong for both models. On AR724x/AR933x the largest frame
size is 2047 bytes, on the AR934x it is 16383 bytes.

Make the MTU setup code model specific, and use the correct
bitmask for both models. Also change the value to the maximum.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39148