bnxt_en: Adjust timer based on ethtool stats-block-usecs settings.

The driver gathers statistics using 2 mechanisms.  Some stats are DMA'ed
directly from hardware and others are polled from the driver's timer.
Currently, we only adjust the DMA frequency based on the ethtool
stats-block-usecs setting.  This patch adjusts the driver's timer
frequency as well to make everything consistent.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>