hw/ssi: imx_spi: Correct the burst length > 32 bit transfer logic

hw/ssi: imx_spi: Correct the burst length > 32 bit transfer logic

For the ECSPIx_CONREG register BURST_LENGTH field, the manual says:

0x020 A SPI burst contains the 1 LSB in first word and all 32 bits in second word.
0x021 A SPI burst contains the 2 LSB in first word and all 32 bits in second word.

Current logic uses either s->burst_length or 32, whichever smaller,
to determine how many bits it should read from the tx fifo each time.
For example, for a 48 bit burst length, current logic transfers the
first 32 bit from the first word in the tx fifo, followed by a 16
bit from the second word in the tx fifo, which is wrong. The correct
logic should be: transfer the first 16 bit from the first word in
the tx fifo, followed by a 32 bit from the second word in the tx fifo.

With this change, SPI flash can be successfully probed by U-Boot on
imx6 sabrelite board.

  => sf probe
  SF: Detected sst25vf016b with page size 256 Bytes, erase size 4 KiB, total 2 MiB

Fixes: c906a3a01582 ("i.MX: Add the Freescale SPI Controller")
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210129132323.30946-10-bmeng.cn@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>