bnx2x: Prevent false warning for lack of FC NPIV

Not all adapters have FC-NPIV configured. If bnx2fc is used with such an
adapter, driver would read irrelevant data from the the nvram and log
"FC-NPIV table with bad length..." In system logs.

Simply accept that reading '0' as the feature offset in nvram indicates
the feature isn't there and return.

Reported-by: Andrew Patterson <andrew.patterson@hpe.com>
Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>