nfp: clean up return types in kdoc comments

Remove 'Return:' information from functions which no longer
return a value.  Also update name and return types of nfp_nffw_info
access functions.

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>