mlxsw: i2c: Add missing field documentation

Suppress following warning when compiling with W=1:

drivers/net/ethernet/mellanox/mlxsw//i2c.c:78: warning: Function
parameter or member 'cmd' not described in 'mlxsw_i2c'
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlxsw/i2c.c

@@ -53,6 +53,7 @@
 
 /**
  * struct mlxsw_i2c - device private data:
+ * @cmd: command attributes;
  * @cmd.mb_size_in: input mailbox size;
  * @cmd.mb_off_in: input mailbox offset in register space;
  * @cmd.mb_size_out: output mailbox size;