bnxt_en: Allow the user to set ethtool stats-block-usecs to 0.

For debugging purpose, it is sometimes useful to disable periodic
port statistics updates, so that the firmware logs will not be
filled with statistics update messages.

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