musb_hdrc: add heuristic to make ethernet peripherals use RNDIS RX mode

When you're willing to trust the USB host not to commit certain errors, the
CPPI "rndis" RX mode can be used with the peripheral side g_ether driver.
(In other cases, rndis rx mode tends to misbehave _very_ rudely ...)

This patch adds a heuristic to detect when the safe scenario is likely, and
a module parameter to disable the heuristic in contexts when it's likely
to fail; for example, users of gadgetfs will likely need to disable it.

This gave about a 60% improvement in throughput with CDC Ethernet (!!) and a
slight reduction in CPU load.  That's measured with TTCP, which pushes a lot
of full size Ethernet frames and thus can see three IRQs per packet reduced
to just one, and also get I/O overlap when double buffering packets.  That is,
net 10 MByte/sec sustained, using double buffering but no DMA transfer queues;
it's unclear why TX is still just 6 MByte/sec.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>