net/mlx4: rename two constants

MAX_DESC_SIZE is really the size of the bounce buffer used
when reaching the right side of TX ring buffer.

MAX_DESC_TXBBS get a MLX4_ prefix.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>