net/mlx5: FPGA, return -EINVAL if size is zero

Currently, if a size of zero is passed to
mlx5_fpga_mem_{read|write}_i2c()
the "err" return value will not be initialized, which triggers gcc
warnings:

[..]/mlx5/core/fpga/sdk.c:87 mlx5_fpga_mem_read_i2c() error:
uninitialized symbol 'err'.
[..]/mlx5/core/fpga/sdk.c:115 mlx5_fpga_mem_write_i2c() error:
uninitialized symbol 'err'.

fix that.

Fixes: a9956d35

 ('net/mlx5: FPGA, Add SBU infrastructure')
Signed-off-by: Kamal Heib <kamalh@mellanox.com>
Reviewed-by: Yevgeny Kliteynik <kliteyn@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>