sfc: Fix MCDI structure field lookup

The least significant bit number (LBN) of a field within an MCDI
structure is counted from the start of the structure, not the
containing dword.  In MCDI_ARRAY_FIELD() we need to mask it rather
than using the usual EFX_DWORD_FIELD() macro.

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