mlx5: fix mlx5i_grp_sw_update_stats() stack usage

The mlx5e_sw_stats structure has grown to the point of triggering
a warning when put on the stack of a function:

mlx5/core/ipoib/ipoib.c: In function 'mlx5i_grp_sw_update_stats':
mlx5/core/ipoib/ipoib.c:136:1: error: the frame size of 1028 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

In this case, only five of the structure members are actually set,
so it's sufficient to have those as separate local variables.
As en_rep.c uses 'struct rtnl_link_stats64' for this, just use
the same one here for consistency.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>