spi: atmel_qspi: fix race condition in transfer completion check

spi: atmel_qspi: fix race condition in transfer completion check

In atmel_qspi_transfer(), the status register is polled with:

  imr = QSPI_SR_INSTRE | QSPI_SR_CSR;
  return readl_poll_timeout(aq->regs + QSPI_SR, sr,
                            (sr & imr) == imr,
                            ATMEL_QSPI_TIMEOUT);

However, this is racy: QSPI_SR_INSTRE can be set before QSPI_SR_CSR,
and will then be cleared by the read. If that happens, the condition
"(sr & imr) == imr" can never be true, and the function times out.

This race condition is avoided in at91bootstrap by accumulating the
status bits across reads until both bits have been observed:

  /* Poll INSTruction End and Chip Select Rise flags. */
  imr = (QSPI_SR_INSTRE | QSPI_SR_CSR);
  sr = 0;
  do {
    udelay(1);
    sr |= qspi_readl(qspi, QSPI_SR) & imr;
  } while ((--timeout) && (sr != imr));

Update U-Boot's atmel_qspi_transfer() to use the same pattern,
ensuring that both flags are observed even if they are not set
simultaneously.

Signed-off-by: Ramin Moussavi <lordrasmus@gmail.com>
[eugen.hristev@linaro.org: remove 'sr' and fix commit msg]
Signed-off-by: Eugen Hristev <eugen.hristev@linaro.org>