net/mlx5: clean function declarations in eswitch.c up

net/mlx5: clean function declarations in eswitch.c up

We get 2 warnings when building kernel with W=1:
drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads.c:463:5: warning: no previous prototype for 'esw_offloads_init' [-Wmissing-prototypes]
drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads.c:521:6: warning: no previous prototype for 'esw_offloads_cleanup' [-Wmissing-prototypes]

In fact, both functions are declared in
drivers/net/ethernet/mellanox/mlx5/core/eswitch.c,but should be
declared in a header file, thus can be recognized in other file.

So this patch moves the declarations into
drivers/net/ethernet/mellanox/mlx5/core/eswitch.h

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>