sfc: Rationalise MCDI buffer accessors

Add _MCDI_DWORD() which yields an lvalue for the given dword field
and change MCDI_DWORD(), MCDI_SET_DWORD() and MCDI_QWORD() to use it.

Fold the rather trivial MCDI_PTR2() into MCDI_PTR() and _MCDI_DWORD().

Remove MCDI_SET_DWORD2() and MCDI_QWORD2().  MCDI_DWORD2() should also
go, but it still has one user which we'll get rid of later.

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