SPI flash instruction format
| cmd_en | Enable sending of command |
| addr_len | Number of address bytes (0 to 4) |
| pad_cnt | Number of dummy cycles |
| cmd_proto | Protocol for transmitting command |
| addr_proto | Protocol for transmitting address and padding |
| data_proto | Protocol for receiving data bytes |
| cmd_code | Value of command byte |
| pad_code | First 8 bits to transmit during dummy cycles |