It fix coverity issue:
CID 124557 (#1 of 1): Unchecked return value (CHECKED_RETURN)
check_return: Calling rte_eth_tx_buffer_set_err_callback without
checking return value (as is done elsewhere 6 out of 7 times).
Fixes: d6c99e62c852 ("ethdev: add buffered Tx")
Signed-off-by: Slawomir Mrozowicz <slawomirx.mrozowicz@intel.com>