i2c: designware: Fix slave state machine for sequential reads

Some read types from I2C bus don't work correctly when testing the
i2c-designware-slave.c with the slave-eeprom backend. The same reads
work correctly when testing with a real 24c02 EEPROM chip.

In the following tests an i2c-designware-slave.c instance with the
slave-eeprom backend is configured to act as a simulated 24c02 at
address 0x65 on an I2C host bus 6:

1. i2cdump -y 6 0x65 b (OK)
   Random read. Each byte are read using a byte address write with a
   current address read in a same message.
2. i2cdump -y 6 0x65 c (OK, was NOK before commit 3b5f7f10 when it
                        was repeating the 1st byte)
   Repeated current address read. One byte address write message
   followed by repeated current address read messages.
3. i2cdump -y 6 0x65 i (NOK, each 32 byte block repeats the 1st byte of
                        block)
   Sequential read using SMBus Block Read. For each 32 byte block a byte
   address write followed by 32 sequental reads in a same message.

These findings are explained because the implementation has had a
mismatch between hardware interrupts and what I2C slave events should be
sent after those interrupts. Despite that the case 1 happened to have
always the I2C slave events sent to a right order with a right data
between backend and the I2C bus.

Hardware generates the DW_IC_INTR_RD_REQ interrupt when another host is
attempting to read and for sequential reads after. DW_IC_INTR_RX_DONE
occurs when host does not acknowledge a transmitted byte which is an
indication the end of transmission.

Those interrupts do not match directly with I2C_SLAVE_READ_REQUESTED and
I2C_SLAVE_READ_PROCESSED events which is how the code was and is
practically using them. The slave-eeprom backend increases the buffer
index with the I2C_SLAVE_READ_PROCESSED event and returns the data from
current index when receiving only the I2C_SLAVE_READ_REQUESTED event.

That explains the repeated bytes in case 3 and also case 2 before
commit 3b5f7f10 ("i2c: designware: slave should do WRITE_REQUESTED
before WRITE_RECEIVED").

Patch fixes the case 3 while keep cases 1 and 2 working with following
changes:

- First DW_IC_INTR_RD_REQ interrupt will change the state machine to
  read in progress state, send I2C_SLAVE_READ_REQUESTED event and
  transmit the first byte from backend
- Subsequent DW_IC_INTR_RD_REQ interrupts will send
  I2C_SLAVE_READ_PROCESSED events and transmit next bytes from backend
- STOP won't change the state machine. Otherwise case 2 won't work since
  we cannot distinguish current address read from sequentiel read
- DW_IC_INTR_RX_DONE interrupt is needless since there is no mechanism
  to inform it to a backend. It cannot be used to change state machine
  at the end of read either due the same reason than above
- Next host write to us will change the state machine from read to write
  in progress state
- STATUS_WRITE_IN_PROGRESS and STATUS_READ_IN_PROGRESS are considered
  now to be status flags not the state of the driver. This is how we
  treat them in i2c-designware-master.c

While at it do not test the return code from i2c_slave_event() for
I2C_SLAVE_READ_REQUESTED and I2C_SLAVE_READ_PROCESSED since it returns
always 0.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>