qlcnic: dcb: default to returning -EOPNOTSUPP

Clang static analysis reports this issue
qlcnic_dcb.c:382:10: warning: Assigned value is
  garbage or undefined
  mbx_out = *val;
          ^ ~~~~

val is set in the qlcnic_dcb_query_hw_capability() wrapper.
If there is no query_hw_capability op in dcp, success is
returned without setting the val.

For this and similar wrappers, return -EOPNOTSUPP.

Fixes: 14d385b9

 ("qlcnic: dcb: Query adapter DCB capabilities.")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>