iw_cxgb3: Fix incorrectly returning error on success

[ Upstream commit 67f1aee6f45059fd6b0f5b0ecb2c97ad0451f6b3 ]

The cxgb3_*_send() functions return NET_XMIT_ values, which are
positive integers values. So don't treat positive return values
as an error.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>