net/mlx5e: Unindent the else-block in mlx5e_xmit_xdp_buff

The next commit will add more indentation levels to mlx5e_xmit_xdp_buff.
To keep indentation minimal, unindent the else-block of the if-statement
by doing an early return.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>