sfc: Use explicit bool for boolean variables, parameters and return values

Replace (cond ? 1 : 0) with cond or !!cond as appropriate, and
(cond ? 0 : 1) with !cond.

Remove some redundant boolean temporaries.

Rename one field that looks like a flag but isn't.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>