In tx_stats debugfs, txrate->flags may contain multiple bits. For
example, RATE_INFO_FLAGS_SHORT_GI could be set, and tx stats bytes
and packets will be not updated correctly.

Fix this issue by using bit operation to check txrate->flags.

Tested HW: QCA9984
Tested Firmware: 10.4-3.9.0.1-00007
Signed-off-by: Brandon Huang <yanghuan@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>