net/mlx5: Delete unused FPGA QPN variable

fpga_qpn was assigned but never used and compilation with W=1
produced the following warning:

drivers/net/ethernet/mellanox/mlx5/core/fpga/core.c: In function _mlx5_fpga_event_:
drivers/net/ethernet/mellanox/mlx5/core/fpga/core.c:320:6: warning:
variable _fpga_qpn_ set but not used [-Wunused-but-set-variable]
  u32 fpga_qpn;
      ^~~~~~~~

Fixes: 98db16ba

 ("net/mlx5: FPGA, Handle QP error event")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>