cxgbe(4): More fixes for the T6 FCS error counter.

r365732 was the first attempt to get an accurate count but it was
writing to some read-only registers to clear them and that obviously
didn't work. Instead, note the counter's value when it is supposed to
be cleared and subtract it from future readings.

dev.<port>.stats.rx_fcs_error should not be serviced from the MPS
register for T6.

The stats.* sysctls should all use T5_PORT_REG for T5 and above. This
must have been missed in the initial T5 support years ago. Fix it while
here.

MFC after: 3 days
Sponsored by: Chelsio Communications