cxgbe(4): Add flag to reliably stop the driver from accessing hw stats.

There are two kinds of routines in the driver that read statistics from
the hardware: the cxgbe_* variants read the per-port MPS/MAC registers
and the vi_* variants read the per-VI registers. They can be called
from the 1Hz callout or if_get_counter. All stats collection now takes
place under the callout lock and there is a new flag to indicate that
these routines should not access any hardware register.

MFC after: 2 weeks
Sponsored by: Chelsio Communications