When processing a received packet, only sync the amount of bytes

When processing a received packet, only sync the amount of bytes
mcx(4) told us has arrived.  The DMA map's mapsize on RX packets
is the length of the allocated buffer.  For mcx(4), this can be
more than around 9000 bytes, as each buffer will be at least as
big as the maximum supported MTU.  There's no need to sync the
whole buffer, if it's only a small packet.

ok dlg@ jmatthew@