bnxt_en: Fix ethtool selftest output in one of the failure cases

When RDMA driver is loaded, running offline self test is not
supported and driver returns failure early. But it is not clearing
the input buffer and hence the application prints some junk
characters for individual test results.

Fix it by clearing the buffer before returning.

Fixes: 895621f1 ("bnxt_en: Don't support offline self test when RoCE driver is loaded")
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>