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: a9956d35d199 ('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>

net/mlx5: FPGA, Add SBU infrastructure

Add interface to initialize and interact with Innova FPGA SBU
connections.
A client driver may use these functions to set up a high-speed DMA
connection with its SBU hardware logic, and send/receive messages
over this connection.

A later patch in this patchset will make use of these functions for
Innova IPSec offload in mlx5 Ethernet driver.

Add commands to retrieve Innova FPGA SBU capabilities, and to
read/write Innova FPGA configuration space registers and memory,
over internal I2C.

At high level, the FPGA configuration space is divided such:
0x00000000 - 0x007fffff is reserved for the SBU
0x00800000 - 0xffffffff is reserved for the Shell
0x400000000 - ... is DDR memory

A later patchset will add support for accessing FPGA CrSpace and memory
over a high-speed connection. This is the reason for the ACCESS_TYPE
enumeration, which currently only supports I2C.

Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>