cxgbe(4): Tidy up PAUSE frame accounting.

Figure out if the chip is counting PAUSE frames in the "normal" stats
and take them out if it is. This fixes a bug in the tx stats because
the default hardware behavior is different for Tx and Rx but the driver
was treating both the same way. The result was that OPACKETS, OBYTES,
and OMCASTS were under-reported (if tx_pause > 0) before this change.

Note that the mac_stats sysctl still gives you the raw value of these
statistics straight from the device registers.