net/mlx5: FPGA, make mlx5_fpga_device_brb static

Fix warning when building with -Wall:
drivers/net/ethernet/mellanox/mlx5/core/fpga/core.c:105:5: warning: symbol 'mlx5_fpga_device_brb' was not declared. Should it be static?

Fixes: c43051d7 ("net/mlx5: FPGA, Add SBU bypass and reset flows")
Reported-by: Or Gerlitz <gerlitz.or@gmail.com>
Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>