sfc: Combine high-level header files

All files that include ethtool.h, rx.h or tx.h are also including
efx.h, and there is no good reason to separate out the few
declarations they contain.  Therefore fold them into efx.h.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>