mtd: spi-nor: intel-spi: Fix atomic sequence handling

On many older systems using SW sequencer the PREOP_OPTYPE register
contains two preopcodes as following:

  PREOP_OPTYPE=0xf2785006

The last two bytes are the opcodes decoded to:

  0x50 - Write enable for volatile status register
  0x06 - Write enable

The former is used to modify volatile bits in the status register. For
non-volatile bits the latter is needed. Preopcodes are used in SW
sequencer to send one command "atomically" without anything else
interfering the transfer. The sequence that gets executed is:

  - Send preopcode (write enable) from PREOP_OPTYPE register
  - Send the actual SPI command
  - Poll busy bit in the status register (0x05, RDSR)

Commit 8c473dd6 ("spi-nor: intel-spi: Don't assume OPMENU0/1 to be
programmed by BIOS") enabled atomic sequence handling but because both
preopcodes are programmed, the following happens:

  if (preop >> 8)
  	val |= SSFSTS_CTL_SPOP;

Since on these systems preop >> 8 == 0x50 we end up picking volatile
write enable instead. Because of this the actual write command is pretty
much NOP unless there is a WREN latched in the chip already.

Furthermore we should not really just assume that WREN was issued in
previous call to intel_spi_write_reg() because that might not be the
case.

This updates driver to first check that the opcode is actually available
in PREOP_OPTYPE register and if not return error back to the spi-nor
core (if the controller is not locked we program it now). In addition we
save the opcode to ispi->atomic_preopcode field which is checked in next
call to intel_spi_sw_cycle() to actually enable atomic sequence using
the requested preopcode.

Fixes: 8c473dd6 ("spi-nor: intel-spi: Don't assume OPMENU0/1 to be programmed by BIOS")
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>