net/mlx5: fix an uninitialized variable

net/mlx5: fix an uninitialized variable

Since patch "mbuf: structure reorganization" the compiler complains
sometimes (in some conditions):

 .../drivers/net/mlx5/mlx5_rxtx.c: In function ‘mlx5_rx_burst’:
 .../drivers/net/mlx5/mlx5_rxtx.c:2082:17: error: ‘len’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]

len is not initialised as it will be at the first segment of a received
packet, but it remains hard for the compiler to determine it.

Fixes: 9964b965ad69 ("net/mlx5: re-add Rx scatter support")
Cc: stable@dpdk.org
Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>