The RMFT_DEST_MAC and TMFT_SRC_MAC register fields were previously
documented as 44 bits wide, whereas a MAC address has 48 bits.
Thankfully the hardware uses the correct width and the driver has
used separate definitions that divide each of these into 32-bit and
16-bit fields.

Fix the initial definitions for these fields and rewrite the latter
definitions to use them.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>