if_awg: don't process transmitted packets on TX_BUF_UA_INT, only on TX_INT

TX_BUF_UA_INT is set when there are no buffers to transmit and can
happen before hw.awg.tx_interval segments have been transmitted.

To reduce load, tx cleanup should be done in hw.awg.tx_interval intervals.

Submitted by: Guy Yur <guyyur@gmail.com>
Differential Revision: https://reviews.freebsd.org/D13034