ieee1394: eth1394: clean up fragment_overlap

offset > fi->offset + fi->len - 1  ==  !(offset < fi->offset + fi->len)
offset + len - 1 < fi->offset      ==  !(offset + len > fi->offset)
!(A || B)  ==  (!A && !B)

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>