mlxsw: spectrum: Load firmware version based on devlink parameter

Load firmware version based on 'fw_load_policy' devlink parameter. The
driver supports these two options:
    * DEVLINK_PARAM_FW_LOAD_POLICY_VALUE_DRIVER (0)
      Default, load firmware version preferred by the driver
    * DEVLINK_PARAM_FW_LOAD_POLICY_VALUE_FLASH (1)
      Load firmware currently stored in flash

The second option, 'flash', allow the device to run with different firmware
version than preferred by the driver for testing and/or debugging purposes.
For example, testing a firmware bug fix.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>