bnxt_en: Fix allocation of zero statistics block size regression.

Recent commit added logic to determine the appropriate statistics block
size to allocate and the size is stored in bp->hw_ring_stats_size.  But
if the firmware spec is older than 1.6.0, it is 0 and not initialized.
This causes the allocation to fail with size 0 and bnxt_open() to
abort.  Fix it by always initializing bp->hw_ring_stats_size to the
legacy default size value.

Fixes: 4e748506 ("bnxt_en: Allocate the larger per-ring statistics block for 57500 chips.")
Reported-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Tested-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>