bnxt_en: Reduce default ring sizes.

The current default tx ring size of 512 causes an extra page to be
allocated for the tx ring with only 1 entry in it.  Reduce it to
511.  The default rx ring size is also reduced to 511 to use less
memory by default.

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