sfc: Fill out the set of MCDI accessors

We need to access arrays of 16-bit words and 32-bit dwords in MCDI
buffers based on the MCDI protocol definitions.

We should also be able to read and write fields within structures,
without specifying an array index each time.  So add MCDI_FIELD()
and make MCDI_ARRAY_FIELD() use it.  Also add MCDI_SET_FIELD().

Split MCDI_ARRAY_PTR() into MCDI_ARRAY_STRUCT_PTR() and
_MCDI_ARRAY_PTR(), which are currently identical but will diverge in
later changes.

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