firewire: net: fix maximum possible MTU

Commit b3e3893e ("net: use core MTU range checking in misc drivers")
mistakenly introduced an upper limit for firewire-net's MTU based on the
local link layer controller's reception capability.  Revert this.  Neither
RFC 2734 nor our implementation impose any particular upper limit.

Actually, to be on the safe side and to make the code explicit, set
ETH_MAX_MTU = 65535 as upper limit now.

(I replaced sizeof(struct rfc2734_header) by the equivalent
RFC2374_FRAG_HDR_SIZE in order to avoid distracting long/int conversions.)

Fixes: b3e3893e

('net: use core MTU range checking in misc drivers')
CC: netdev@vger.kernel.org
CC: linux1394-devel@lists.sourceforge.net
CC: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>