nfp: core: make scalar CPP helpers fail on short accesses

Currently the helpers for accessing 4 or 8 byte values over
the CPP bus return the length of IO on success.  If the IO
was short caller has to deal with error handling.  The short
IO for 4/8B values is completely impractical.  Make the
helpers return an error if full access was not possible.
Fix the few places which are actually dealing with errors
correctly, most call sites already only deal with negative
return codes.

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>