nfp: core: allow 4-byte aligned accesses to Memory Units

Current code doesn't enforce length requirements on 32bit accesses
with action NFP_CPP_ACTION_RW to memory units, but if the access
is only aligned to 4 bytes as well we will fall into the explicit
access case and error out.  Such accesses are correct, allow them
by lowering the width earlier.

While at it use a switch statement to improve readability.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>