sfc: Remove bogus comment about MTU change and RX buffer overrun

RX DMA is limited by the length specified in each descriptor and not
by the MAC.  Over-length frames may get into the RX FIFO regardless of
the MAC settings, due to a hardware bug, but they will be truncated by
the packet DMA engine and reported as such in the completion event.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>